I'm the king of Acid no lie.
I been using it 4ever & even it do vocals in it now.
I use to make beats only, next came remixes, then hey I know it so well why not go ahead recording vocals?
I've used damn near everyday for 8 or 9 years it seems like.

Now I'm not bragging saying my music is all that,
just saying I know how to use Acid as good as any1.

1st off...
Kill Sound Forge, it is not needed. I did this same setup at 1st as well. SF is good for mastering a whole complete song, but just an extra 4 you at this point.
There may be a rare situation where you need 2 master your sample 1st, but Acid supports the same fx plug-ins.

Now take the complete song & just drag it in Acid.
Beatmapper comes automatic.
If you know the part you want I usually set the "down beat" to that bar. If it is an extremely electronic sample you will find the exact tempo & the whole song is well... beatmapped.
More than likely you will get the thing to stay on beat for a few bars & slowly drift off.
I use a click track & just right click "split" on the sample now edit the start point where it begins 2 drift to kind of resnap it.
There is no magic bullet to get a whole song to fit on perfect tempo all the way thru w/o doing this unless it is a newer electronic tempo song.
Good news is it is fast as hell to split the og track & resnap because you found the basic tempo w/ the beatmapper.
Also, it is rare you want to snap the whole song so grabbing the parts needed will only take minutes or seconds after some practice.

You probably have seen by now that if you pick beatmapping the pitch changes w/ tempo. It doesn't when loop is selected.
Beatmapped mimicks that by checking the preserve pitch box under track properties. If you throw in the whole song & pick change to loop bye bye ram & flexability.

I use to beatmap as described above save the clips as short loops.
Now I lean towards just beatmapping the whole song perfect to the click track by using multiple "track splits" & adjusting each segments start point. Then I save the whole thing as a project leaving me w/ any part of the original song ready to go at any speed or any pitch in the future.

I just sync in Acid then throw into my sampler currently MV8800 for chopping. I like the hands on feel, but splitting the track like I said is really chopping it. The same rules apply just cut it down more. To chop it in Acid I do from time to time in post-production.


Sorry if that is hard to follow, I'm not great at explaining, but tried to help. 1