is it possible to add/ remove VP from a state through a focus?

Yep! You'll need to know the province ID though, which you can find here: https://hoi4.paradoxwikis.com/List_of_provinces (this page will probably take a bit longer to load than you expect) or by using the Nudge tool in game.

The two rewards you may need are:

add_victory_points = {

province = 1234

value = 10

}

This will add to the current VP value of the province (so middle of Siberia would set the province to 10, but something like London would be set to 60)

And:

set_victory_points = {

province = 1234

value = 10

}

This will set the exact VP value, increase or decrease.