The basic jist of Trollhaunt is that there's a troll leader who wishes to claim the lands of the ancient troll kingdom (which happens to be in the Nentir Vale). The Town of Moonstair is relevant (Moonstair has a gate to the Feywild and is the city closest to the troll's hangout, thus the first place to be conquered). Finally, one of the hooks in the adventure is that the Baron of Therund's son is killed trying to take on the head troll and his ancestral sword left in the warren - PCs are tasked to give the news to the Baron, and the Baron asks them to take vengeance upon the trolls/retrieve his son's body/sword.
So there's a few things here.
1) Integrate the Barony of Therund and the Baron's son Eteran in early, so that his death means something to the PCs. Not in the "high level NPC that saves the day", but a nice guy. Maybe he gives the PCs some of his loot his crew doesn't need, maybe he is passing through and gives the PCs some vital info, maybe he's there in Winterhaven (trying to cross the mountains) and is gearing up to face Kalarel when the PCs return from vanquishing Kalarel (so he is very gracious and honors the PCs' actions). Maybe he's passing through Thunderspire with his party when the PCs are. This way, the PCs are like "They killed Eteran! Those Bastards!" When he dies in Trollhaunt, he's actually on a layover, returning from battling a Cult in a Distant Land (hint hint, CUUUULT).
2) Have Moonstair come in at some point. Just so the PCs have heard of it, have been there, maybe they needed to cross over into the Feywild an that was a reliable, easy way with an established, safe path on the feywild side. It's located in the southwestern part of the Nentir Vale.
3) Drop hints about the ancient Troll Kingdom. I.e. PCs wandering through the forest stumble upon an ancient statue that looks like a troll seated on a throne. They could find a cave mural that has clawed giants with many subjects, eating them in a big celebration. Perhaps the Deva fought in the war against the trolls.
If you are however ending up sandboxing the Nentir Vale, you might want to check out the book "Threats to the Nentir Vale". It is a hook/info goldmine of the region.
Oh. I can also write up my notes on the Parliament of Jagged Crags, the fey court on the Feywild side of Thunderspire, if you want to implement fey in in any way in TL. I figure the Vecna guy would at least have some relationship with fey, given how they love lore/knowledge and Vecna is all about secrets (and since that guy wants to do a coup of the Mages, members of the Court may be for or against that).