I've found a good rule of thumb for vis sources is roughly 3-4 pawns/magus/year for an average covenant. Drop that down to 2 if you want to run vis-poor, or go higher if you want them to be rolling in the stuff.
Regardless of what the initial boons/hooks and build points say, aiming for something in this vicinity over time should stabilise their vis economy. Remember, though, that this includes vis from trade or other sources as well as found in the wild.
If the covenant isn't bringing in that amount, use their role to determine whether they found a renewable vis source. If they roll well, have them find some non-renewable vis as well. Everyone likes a little reward.
If the covenant has already hit their stride with vis income, you can mix things up a bit. Either they don't find anything, or they find a vis source that has complications (claimed by another covenant, protected by a difficult beastie, corrupted, etc). Of course, there's nothing stopping you from throwing the occasional one of these at the players before they hit their vis stride.
I'd personally shy away from basing their success purely on a die roll. Vis isn't something that there's an unlimited amount of - in a vis poor game a successful per+magic lore roll may tell them 'nope, you've tapped out all available sources.'
The other fun thing about vis is it's seasonal, and in many cases it requires some interesting processes to harvest. Simply walking around looking for it may not be enough: you need to talk to the locals, keep an ear out, be in the right season, be under the right stars, etc. So even if you say 'nope' one day, you are perfectly justified saying 'actually... yes!' later.
Because of the intricicies that can involve vis hunting, I'd normally not allow a per+awareness with spot-vis spells to work. I'd instead say a person doing that gets a +1 on their per+magic lore roll, since it makes verification easier. That or it lets you send your mundane magic-lore-monkey out looking for vis under that enchantment and you can stay at home in your lab. Mmmmm, lab.