I have seen the Vector resin canopy set firsthand, and it is truly amazing. The quality control must be terrifyingly exacting; I can only imagine what the reject rate is. How well it fits is something that I cannot comment on, as I have not used it.
It is also available from Sprue Brothers:
http://store.spruebrothers.com/product_p/vecvds48021.htm
Too bad Falcon/Squadron never did a vac canopy set for the Monogram kit. I have two P-61s in the stash and would get vac canopy replacements in a heartbeat.