In bridge, does it make sense to "tighten" or 'loosen" up on bidding depending on vulnerability?
For opening suit bids no, emphatically not. The reason is simple mathematics in that when you open (in 1st or 2nd seat) you have no idea whether your side is looking at part score, game, or slam; and the appropriate aggressiveness towards each of these levels varies differently with vulnerability.

The relative worth of game to part-score goes up with vulnerability so it is appropriate to bid slightly more aggressively towards vulnerable games at IMP (and Rubber, both Four-Deal/Chicago and standard) scoring.  For example, it is appropriate to bid a non-vulnerable game with a 50% chance to make (ie requiring only a finesse) and appropriate to bid a vulnerable game with a 35% chance to make (requiring a finesse and a 3-2 break).

However the relative worth of slam to game goes down when vulnerable, so one should be slightly more cautious bidding vulnerable slams at IMPS. It is now appropriate to bid a non-vulnerable small slam with a 50% chance to make but only appropriate to bid a vulnerable small slam with a 65% chance to make (roughly requiring only a 3-2 break).

Empirically, these percentage differences are often estimated as representing about a 1 or 1.5 point difference in partnership combined strength, when better information is not available.

Third seat openings however can probably discount the possibility of slam when holding minimum values, so here the logic is that one should open slightly more aggressively, not less, when vulnerable, because of the increased relative value of game.

This is all nearly the complete opposite of your suggestion. Only when making a competitive part-score decision or a preemptive opening does the logic of the question hold true.

Therefore, until you know which levels your partnership is aiming for, it is inappropriate to adjust your bidding. This is done by having the inviter, only, make the adjustment and only with the inviting bid. Then the partnership can rely on all other calls being standard, allowing more consistent decisions to be made by the partnership.

As a final note, in regards Grand Slam bidding, Barry Crane (and the Four Aces before him) had a commandment to never bid grand slam in Swiss Teams (my emphasis):


  The caution about not bidding grand slams in Swiss Teams made a lot of sense particularly if
  your opponents are not all that skillful as nothing is more demoralizing than to go down one at
  seven while the other team only reached game. In fact, I remember the original Four Aces of forty
  or more years ago had a rule that if you bid seven once and went down you would be forgiven but
  if you did it the second time you were off the team.




Revisiting, for Matchpoints:

The above discussion is entirely intended to guide bidding in IMPS, Chicago, or Rubber scoring. In Matchpoints, different considerations apply.

In Matchpoints the amount by which you gain or lose compared to the field is irrelevant. Rather the number of times you are plus or minus to the field is paramount. This means that far more decisions should be made purely on a basis of 
whether or not they succeed a straight 50% of the time. 

Consider a touch-and-go game in a major that can be made only on a favourable lead and finding a subtle endplay. Here is a Matchpoint analysis for a case of likelihood for the four cases:

Contract                 Raw Score  
&amp; Result                  NV   V    #   MPs  
Bid 4 making 10 tricks:  420  620   2  11.0  
Bid 3 making 10 tricks:  170  170   2   9.0  
Bid 3 making  9 tricks:  140  140   4   6.0  
Bid 4 making  9 tricks:  -50 -100   4   2.0  


In both case, simply making the 10th trick without bidding it is both sufficient to generate a good to excellent Matchpoint score, and avoids the risk of a terrible one. 

In the nearly 5 decades I have been playing bridge, the quality of my game (at Matchpoints) has almost never been from the number of tops partner and I got - it is almost always a consequence of all the available bottoms that we didn't get.