How significant is military technological advantage? Like, if there is a war between France and vietnamese states in 1870, will it be literally like in history that 500 French could utterly route 10000 Vietnamese? Or more like eu IV style where after certain time all the distant asian nations manage to catch up and fight Europeans on equal terms?
This is a balancing question so it's a bit hard to answer definitively at this point in development, but I can elaborate on
intent: if there's a significant discrepancy between two armies' military capabilities, this discrepancy
can be made up by superiority in numbers
in some cases but it's highly situational, depending on the Generals involved, terrain fought in, focus on offense vs defense, and so on. The biggest factor that would benefit the French in this situation is dynamic battle sizing and combat width mechanics, which makes it much less likely the French would be outnumbered and assaulted 20:1 in
any given battle, making it possible for a small detachment to stand their ground or even conquer territory.
If the question is more about how rapidly countries tend to catch up to the forerunners in terms of technology, that also varies a lot - most of the countries that start at a disadvantage get most of their tech development from spread, but also start with low Literacy and in some cases severe lack of Freedom of Speech, which slow down the rate by which they develop. They're also unlikely to be able to afford well-staffed Universities as a result of their low industrial development and regressive taxation systems. But if a country can overcome these early limitations somehow they can certainly catch up to the global technology leaders.
Finally, it bears mentioning that in Victoria 3 most technologies give you more options, not global buffs - and those options usually cost you something. So it's rarely the case that acquiring military tech just makes your army better, you have to implement and pay ongoing costs for the upgrades, too. If you want to compete with the Great Powers you can't just rely on passive assimilation of their innovations.