It seems to me that it's supposed to be a rare event - but it's not. So here's my slightly modified event, just copy paste it to whatever file original event is in
Note, I don't have VV, so please report any problems 
Basic changes are increasing triggers - that one wants to be a ruler is not enought. One needs to be wealthy and important enought as well. Because of this I increased needed wealth to 1000 (original EU:R standard for "wealthy") and prominence to 75.
I also changed modifiers. If character wants to be a ruler and have means to do so, it will pursue it more often - hence decreased time with more prominence & wealth.
Finally, effects. The heirs weren't so defenceless, so the chance that something happens to ruler is decreased (75->50%); the chance that something happens to triggering character is increased to 50%, and now there's a chance that triggering character will die, too. Heir is supposed to have a lot of influence, so giving him equal chances to survive seems to be a fair deal.
character_event = {
id = 7515
trigger = {
has_objective = obj_become_ruler
is_primary_heir = no
wealth = 1000
prominence = 75
country = {
government = monarchy
}
}
mean_time_to_happen = {
months = 60
modifier = {
traits = ambitious
factor = 0.5
}
modifier = {
traits = content
factor = 1.5
}
modifier = {
prominence = 100
factor = 0.7
}
modifier = {
prominence = 90
factor = 0.8
}
modifier = {
prominence = 80
factor = 0.9
}
modifier = {
wealth = 10000
factor = 0.7
}
modifier = {
wealth = 5000
factor = 0.8
}
modifier = {
wealth = 2500
factor = 0.9
}
modifier = {
loyalty = 90
factor = 1.5
}
modifier = {
loyalty = 80
factor = 1.3
}
modifier = {
loyalty = 70
factor = 1.1
}
modifier = {
NOT = { loyalty = 35 }
factor = 0.9
}
modifier = {
NOT = { loyalty = 25 }
factor = 0.9
}
modifier = {
NOT = { loyalty = 15 }
factor = 0.9
}
modifier = {
from_ruler_family = no
factor = 0.5
}
}
title = "EVTNAME7515"
desc = "EVTDESC7515"
option = {
name = "EVTOPTA7515"
wealth = -25
random_list = {
30 = {
country = {
random_character = {
limit = {
is_primary_heir = yes
}
death = yes
}
}
}
20 = {
country = {
random_character = {
limit = {
is_primary_heir = yes
}
add_trait = maimed
}
}
}
20 = {
prominence = -10
loyalty = -10
popularity = -10
wealth = -500
}
30 = {
death = yes
}
}
}
}
Basic changes are increasing triggers - that one wants to be a ruler is not enought. One needs to be wealthy and important enought as well. Because of this I increased needed wealth to 1000 (original EU:R standard for "wealthy") and prominence to 75.
I also changed modifiers. If character wants to be a ruler and have means to do so, it will pursue it more often - hence decreased time with more prominence & wealth.
Finally, effects. The heirs weren't so defenceless, so the chance that something happens to ruler is decreased (75->50%); the chance that something happens to triggering character is increased to 50%, and now there's a chance that triggering character will die, too. Heir is supposed to have a lot of influence, so giving him equal chances to survive seems to be a fair deal.