You cannot increase attributes after turning vampire, so you're stuck with whatever you have.
But still, it's unlikely it's any number besides 5000 at that point, anyway, and it's impossible to raise it past that by any means.
Simply put, you just plain can't punch holes in Bronze Colossi with a humanoid that weighs less than a ton. DF just isn't meant to be that kind of game, and BCs are MEANT to be extremely difficult to kill.
Yes, I know being vampire stops further attribute gains and I know DF isn't THAT kind of game, but what I ask is very very simple. Just, JUST tell me how, IF possible a way to cheat and increase the attribute cap of just one adventurer without having to alter the attribute cap of an entire race. That's it.
I probably won't be able to punch a hole through a bronze colossus even if I have a strength value of over 5000, but c'mon. I just wanna experiment and have fun.
5000 is the max of everything in the entire game. It's blanketed over all creatures for every attribute and isn't decided on a race by race basis. This is seemingly unchangeable, or at least I've never seen somebody change it before through modding.
A adventurer's strength cap, as mentioned earlier (if accurate, ain't in the mood for fact checking) is normally 4000 and the only reason you can beat that is because of being a vampire, so technically you've already "cheated" as much as possible. Hell, your the living embodiment of the concept of physical strength at that point.
But...
The colossus is around 300x as big as you, and I wouldn't be surprised if it's strength was in the 4000-5000 range as well, but it's sheer size makes that irrelevant unless it had very, very, little strength. Size has A LOT to do with how easy it is to push things around and the amount of force behind your punches, so if you want to beat the bronze colossus with wrestling/fists like it's nothing, you'll have to increase in size. I don't think you can do that without changing everybody your race, though, but I've not checked it make sure.