It's better than you think.
And worse than you think.
I say this because (in most builds) Vulnerability has taken a back seat to Might. They basically do the same thing right, except Might does it better? ;)
The main difference however, is that Vulnerability is like a low powered AoE Might that everyone in your group continually spams on themselves.
Vulnerability now does +1% damage per stack and maxes out at 25 stacks for 25% damage.
Might gives you +35 Power at level 80 and also stacks up to 25 for +875 Power.
At this point the community doesn't know the exact damage formula (contact me if you do!), but most people agree that it includes a ratio of Attack to Defense. Attack is made up of Power and Weapon Strength while Defense is made up of Toughness and Armor.
This means that the percent damage gained from Might depends on your base values and begins to be less effective than vulnerability as this Attack to Defense Ratio goes up. Unfortunately since we don't know the exact formula I can't tell you the point at which Vulnerability surpasses Might. It may even be out of the range of feasible values for launch characters.
In general, vulnerability gets better as your opponents Toughness goes down and your Weapon Damage/Power goes up.
Also, keep in mind that vulnerability is a debuff on a player and therefore damage from other players is also increased. At level 80 base PvP weapons and Power ranges, just having two players will make vulnerability better than might. So having 5 players able to focus one target has the potential to do +25% damage per player. That's a lot better than just you having might.
And my last point about vulnerability, and why it's worse than you think, is that it can easily be removed by every profession, unlike Might which requires certain skills that only some professions have. Everyone can remove conditions but not everyone can strip boons. So if you are going to run on vuln, make sure you can stack it all the time.
We have two ways to stack Might. Elixir B and HGH. Even if all of our skills are elixirs, stacking Might is tough.
On the other hand, using Utility Goggles and Steel Packed Powder, Engineers can stack vulnerability pretty quickly onto one target.
So even though Might is going to be better for 1v1 or small groups where you know Vulnerability will be removed, I don't think we'll see any Might builds for Engineers. We will however see Grenadier Vulnerability builds.