Hard to say , after they nerfed all the quests exp at vet ranks . it may be faster to just grind trash mobs your lvl.

No extra XP's from being underleveled, at least not explicitly.

You might get more XP's from fighting harder mobs (though you'll have to work harder too, so time-wise, not always to your benefit), but you won't get more XP's than you would fighting them later.

As long as the mobs/quests are withing 5 levels of your character, you'll get XP's and drops. Anything more than 5 levels below you, may as well skip entirely, unless you're going for your armor repair achievement or testing skill combos.

Always best to do the Yellow's first, Green's next, and Grey's if your quest log is full, or close to it. (Grey's will be more than 5 levels below).

You'll always get quest completion rewards and XP's, but it will be scaled to the quest level. Save a starter zone quest until you're VR10 and you're still only gonna get a Level 1 reward from it.

Regarding VR levels, if you look a couple patch notes back, you get diminishing returns on mobs lower than you are, but you still get something.

Quest wise, prior to 1.5, you actually got more VP's being higher level and turning in the same quest as someone that was lower VR turning in the same quest.

Presumably this was to help offset the increase in VP's required for the higher level requirement.

Since VP's are not longer a factor and all ranks take the same amount to level (a cool million), the rank up quest XP's are probably no longer a factor.

Just run the quests in the zones as you go, you'll end up being overleveled as it is if you complete all the content. No reason to try to gain it faster.

Good luck!