1. do enemies and npcs respawn at all? If so where are some good places where things respawn

2 do plants regrow?

3 an extra question... is there a misc vender that sells scrap metal/electronics wonderglue turpentine ...etc?

NPC's respawn, only if they are creatures. (No named NPC's.)

Plants do not as far as I am aware.

No.

yes npc's and enemies respawn but NPC's that have names or are related to quest don't respawn (if you kill them). everything that is respawnable respawns every 3 days i game

yes plants respawn

Traveling Merchants usually sell misc items

Try Old Lady Gibson or Miguel's Pawn Shop.

This

The gun runners also sell a few "scrap items" (especially items that typically help in the creation of weapon repair kits)

Chet in Goodsprings also sells a lot of miscellaneous stuff, as does Johnson Nash in Primm.

I've had decent luck getting such items from Blake in the Crimson Caravan compound as well, however his inventory has an annoying tendency to not refresh so I don't rely on him for such supplies.

Enemies respawn? Is that something that's been fixed/changed with a patch? I played the game when it first came out pre patch and the roads got pretty empty once you clear the baddies out one time. Hardly any more random attacks while traveling.

Enemies in vaults/caves still don't respawn far as I know, but I haven't played those areas (& then went back to check) in a while.

Respawns are largely limited to the above ground, open areas. Animals, viper bandits, some legion/ncr here and there, the fiends of course. Things like that bridge/merchant/legion near Novac aren't random, they're scripted and only happen once.