They're very different games. Ultimately, EUIII is a game of expanding your country through diplomacy and warfare. Victoria is a game of the industrial revolution and colonisation. This leads to some notable differences; I'll try to summarise the most important ones as I see them.
1) It is perfectly possible with most countries in Victoria to play the entire game without expanding your country at all, or to expand only by colonising for manpower and strategic resources. It is possible to "win" the game by doing this (via the common definition of finishing the game as one of the top Great Powers). Many countries are set up so that "holding on to what you have" is actually the most viable game goal (the Ottomans come to mind - other than some minor border adjustments with Prussia, Austria is also usually like this) - successfully retaining and industrialising their lands yields more benefits to them than either of them conquering vast tracts of Russia. EUIII isn't really like that at all; as long as you're not overstretching yourself, expansion is always good, and avoiding warfare is an unusual sort of game - think of Victoria as most countries' default playstyle being like playing a colonisation-focused Portugal in EUIII. Victoria isn't a wargame (which is good - the warfare aspect of the game is probably its weakest point) - it's a game about industrial development and being a player in the race to partition Africa and Asia (including being a player by resisting this). The idea of "blobbing", while certainly quite possible for a skilled player, is in many cases actually counterproductive. Ripping holes out of Russia gets you poor lands with a population that hates you and also makes the rest of the world hate you - it's not really an asset.
2) In Victoria, your population is far and away your most important resource. You actually interact directly with your population (divided into blocks of, say, French Jewish Clerks or South German Catholic Farmers, each one called a "pop"); your most important task as a country is to fulfill their needs and wants so that they don't decide to overthrow your government, emigrate to another country, or revolt to secede to/as some other nation. Each population block has its own political attitudes (they might be liberal and want a party in power that has laissez-faire economic policies, for instance, or be socialists favouring full citizenship for everyone in the nation), and balancing your government and policies to make most of the people happy most of the time is usually the most challenging thing for a new player of the game (and I suspect most players can remember an early game or two that never got finished because they managed to piss off seemingly everyone in their country, causing it to rip apart at the seams with revolts with no end in sight). There is no real equivalent to this in EUIII or any other Paradox game, and is the game's most outstanding feature. Everything in Victoria is about population - how to encourage its growth, encourage immigration (or at least discourage emigration), keep them happy, make them rich and educated so that your research speed is high. How you do it as a conservative monarchy is completely different than how to do it a a free democracy, as well.
3) EUIII spans over four hundred years; Victoria spans a single century. Moreover, while some countries certainly have advantages over others in EUIII, the difference is far more dramatic and pronounced in Victoria and you have a lot less time to do things about it. Some players with gamey tactics and an infinite patience for dealing with revolts can make stuff like a Punjab-conquers-the-world game feasible, but playing a normal game of Ethiopia is playing a game where your viable goal is "survive and industrialise enough to be considered a civilised power worthy of being dealt with as an equal to a small European power", not "sinking Great Britain's fleet and taking India from them". Even playing an already "civilised" European power does not usually have usurping Britain's place as dominant world hegemon as a goal (France or Prussia can dream of this once they've smashed the other, but the Netherlands and Portugal aren't going to do it - again, without extremely gamey tactics) Playing Victoria as a conquer-the-world game is likely to be disappointing - it's not really built for it, and it's both harder and less rewarding than EUIII to play it like that.
EUIII is easy to learn but difficult to master; Victoria is hard to learn and hard to master. Coming to it will make you think EUIII is simple and intuitive, which is often hard for people to believe. But for my money, it is the absolute best game Paradox has ever made and well worth the effort. More than any other game in their catalogue, it makes you feel like your country is a real living, breathing entity, and your population are not an abstract label of "Bulgarian" but the absolute lifeblood of the game. Moreover, it makes countries feel wholly different from each other in a way EUIII can't match. The start for every EUIII game is different, but once you've reached a certain size and (if necessary) Westernised, a large Byzantium plays pretty much the same as a large Russia, France, Austria, Persia, Delhi, Japan, etc. Victoria isn't like that (though it's fair to say the process of industrialising is often very similar); the Ottomans will NEVER play like France or Brazil, not in a hundred games, no matter how big you get (and how they get bigger is likely to differ - Brazil can do straightforward conquest in South America if they're so desirous but are actually more likely to benefit from focusing on encouraging immigration, France is likely to expand mostly through colonisation aside from border adjustments with Prussia and Italy, and the Ottomans can reap some decent reward by absorbing the Arabia peninsula but are unlikely to derive much benefit from taking chunks out of Russia). Don't get me wrong - I do love EUIII, and I do play it quite a bit, but Vicky is the game I keep coming back to despite its age, unintuitive interface and some very irksome bugs because it's absolutely unique and I adore it more than any other game Paradox has ever made. If the idea of a game where internal development of industry and dealing directly with your population are more important than warfare appeals to you, I'd say go for it in an instant.
