i've used a mod to finish the final quest (NCR victory) and continue playing(the mod turns the end movie into an actual room that you can fast travel out of) and after attempting to start the Auld Lang Syne quest i managed to get arcade gannon as a companion and talk about the quest but i can't get the others to join, the woman in newvegas doesn't have any relevant dialog options, and the guy in jacobstown doesn't even start a dialog, he just talks about snipers.

(crap wrong section please move to PC issues)

I'd say this needs to be moved to the mods section. The developers aren't responsible for the fact the game may not work as you want, when using a mod to bypass the design of the game.

As to your problem...since I don't know the mod you're using, I don't know what it's supposed to do beyond let you FT out and continue play. Is it supposed to let you continue quests, or just continue running around killing? Name of mod?

after the final battle the main quest is completed and you're sent to the ending movie screen, the credits scroll, and you're sent to the main menu, this mod simply turns that ending movie into a room with a projector showing the movie and an NPC in the background to product the narration voice,after it stops you're allowed to get up walk around and fast travel back to the wasteland, it doesn't change any quests or anything like that, it just changes where you go for the ending so you aren't locked out. so i don't think it would have an effect on the quests(i've completed several after using it) here's the mod: http://www.newvegasnexus.com/downloads/file.php?id=39304

Auld Lang Syne is a quest that sets up for the final battle at hoover. As you have completed that Battle and it's "quest" already it is probably a quest ID issue. As the only purpose to Auld Lang Syne is to get the remnants to the final battle. As the game wasn't designed to go beyond that battle it's not much wonder that some quests might bug out with a mod like that.

i don't think so,even after reseting the quests it still gives the same problem, if the problem was with completing the last quest than the the Auld quest wouldn't even start.