Vocals are tricky...

Especially on the 808. This is an actual singer, and not sample snippets, right? Many will say 'compression' for starts, which the 808 has in its FX section. Compression will bring a voice or instrument to a 'balanced' median, if used properly.
The whisper portions are brought up, and the screaming portions are brought down, theorhetically. My fave is 'fader riding', now known as 'automated mixing'. Again, the 808 is capable of this also. Basically, you push the faders up to a pleasent level, and pull them down right before you start screaming (bare with me on the examples. ) Automation is pretty 'techie', and uses MIDI CC#7 to 'virtually move the faders' for you at the precise moments you define in the song. But we'll save that for later.

For now, and if you're fresh to the 808 - here's the quickest way for results... If you're vocals on Trk A, hold down SHIFT and press the green buttons above trk B, C, D so they're off. Then press MARK, and you should see trk A go black. Hold MARK down and press CLIPBOARD, it'll light red now. Press Clipboard, and you should hear your vocals come back. Hold down Clipboard and press a blank sample pad. Now your vocals are on that pad. Press SAMPLE/BANK button at top of unit, and scroll down menu till you see NORMALISE, and enter. Select the Pad # where you've put your vocals, enter another Pad # that's free for process to go and enter. May take awhile to process, depending on length of track? When done, press new pad that's red now, and voila! Vocals much louder, right?
To get them back to SONG, simply hold SHIFT + STEP (the stop button on the player controls?), select TRK A, Measure 1 (Or cue up to wherever the vocals came in originally?) and enter. You'll see a note in the upper right of screen - rotate dial till it says SAMPLE, and hit the Pad with the 'normalised' vocal track - done! Well, you'll have to exit out of course to hear it.

This is a pretty basic way to explain it, and you may have your vocals out of time when you replace them. There are ways to fix this, but for now I'll say this - Say NO to Save Current Song when you go to eject Zip disk and power down. This way, you'll keep your 'original Trk A' recording AND the improved Sample Pads as well. You can turn MUTE Trk A next time (just hit the green button so it's off), and practice triggering the pad to get your vocals back in on time. (And I won't be guilty of ruining a masterpiece? )

But soon as you get the manual, and this great forum - you're going to be deadly I bet!