i have had many problems using recycle. it is brilliant if you don't want a full acapella for 3 of 4 minutes of a track, and just want a loop of 4 bars say (like a chopped up soul sample would be recorded back over a hip hop beat). but when it comes to full acapellas its really hard to get the tempo to match. some people say you can work out the tempo in recycle, which you can, however it calculates this tempo based on the amount of bars in the track, and if this acapella has a time lag of 1 second before the acapella starts, then this will cause a floor in the calculated tempo. (i'm assuming this lol, as ive experimented a lot with recycle and reason). i think you're best bet is to use something like ableton or acid, like others advise, and rewire it to reason, so you can have you're full song in reason, and just the acapella in ableton for example. this is what i hear you can do, and i'm going to try and get hold of ableton right now lol.

good luck