I recently came back to the game and have been playing 2h frost. What item lvl would be safe to switch to unholy to receive the same results. I know both specs very well but haven't tried unholy since around 480 Item lvl and it was around 10k dps behind frost for me. For now im just asking for tips/advice of what to do from here. Would I be best off waiting for the VP trinket then trying FB again since haste proc wont benefit FB at all?
since you have a feather, it'll be an increase to switch now, as long as you can properly play festerblight (theres a nice guide for it in the stickies).
the VP trink would be a much better choice for FB, yes, but the feather makes the biggest difference.
I played 2h frost pretty much up until I got a Fabled Feather of Ji-kun, though if you want my advice, switch to Traditional Unholy earlier then I did, so you can get used to the rotational differences between Frost and Unholy, but you won't really get a MAJOR dps increase from the Festerblight playstyle until you have a high-strength proc trinket or two. The VP Shado-Pan trinket is a good start, even an LFR Fabled Feather is better then everything normal mode.
However, this is all a moot point, it's really all about what you'd rather play. I enjoy both specs more or less the same, both have things that annoy me about them and things that are really fun and rewarding.
Check the Festerblight 101 sticky up top for better information.