If you've been having a lot of problems completing quests in Northrend that require you to control a siege vehicle (or something similar to a vehicle) you may consider turning off some of your mods. I know, that sounds like a canned GM response to a ticket, right? Well, it's true, and something you may actually want to try. While questing in Northrend, I started to notice a lot of my friends, guildmates, and random people in various channels were complaining about broken quests that I had finished no more than thirty minutes before. How could they all be broken? I just did them! It took awhile, but we eventually figured out it was their mods. Specifically, their mods that modify how your action bars look or work.
In Wrath, Blizzard implemented a new UI and system for their vehicles, and some bar mods haven't been updated to reflect these additions. Or they have, and players haven't updated on their end. As far as mod updates go, this is one of those things that counts as a big deal. If your bar mod is suppressing the vehicle UI or preventing your action bar from switching over to the vehicle's abilities, you're not going to be able to complete a lot of quests, and those ones tend to be the most fun and unique. Naturally, some quests are buggy, but a vast majority of them aren't. If you're consistently running into issues with vehicle combat, please, check your mods for your sake. A lot of the quests are way fun, so before writing them off, check things on your end first.
If you absolutely must have a bar mod and yours isn't Wrath compatible yet, you might want to check out Dominos. There are other updated bar mods, absolutely, but this one seems the most popular recently. If you'd rather stick with something else, just hit Curse or whatever and check for updates. You'll be glad you did.