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Vulnerability is very important in many bridge bidding decisions.
For borderline cases, go "low" if you are vulnerable but be aggressive if not vulnerable.
Opening Bids
Assuming you open most 12-counts, have no qualms about opening 1
with this hand regardless of vulnerability:
K103 
Q1087 
A2 K1087
With all of those 10's and two decent four-card suits, this should be everyone's opening bid.
However, what if we change it to:
Q105 A643 KJ3 Q32
Aggressive players will open this hand no matter what. I recommend opening it if not vulnerable. But pass if vulnerable. If you open and get overboard vulnerable to the tune of -200, it is usually a bad result.
My advice: In borderline cases: Open if WHITE. Pass if RED.
Overcalls
Again, resolve borderline cases based on vulnerability.
RHO opens 1 and you are considering a 2 overcall with:
105 A3 KJ3 K108762
Vulnerable, I'd pass. Not vulnerable, I'd overcall 2. Aside from the risk factor vulnerable (-200, -500, etc.), your partner will expect more from a vulnerable overcall--and might get you too high if you don't have your bid.
Preempts
Everyone has their own personal style (aggressive or conservative). But within that framework, you have to observe the vulnerability. Again, resolve borderline cases by looking at the colors. Everyone would open 2 with:
KQJ1087 43 43 K87
Vulnerability is not an issue.
But, what about:
KQ9765 43 43 Q76
Vulnerable, I wouldn't do it (very aggressive players would). But, not vulnerable, I wouldn't mind opening 2. Note: Position is also relevant (in 2nd position, preempts should be sounder than if you are the dealer or in 3rd seat).