i hope ive explained this right as ive just knocked this 'guide' up in 10 seconds haha but heres how you do what i think it is you mean.
www.emotioncomplex.co.uk/reverb1.JPG
www.emotioncomplex.co.uk/reverb2.JPG
1) Original Sample (i didnt use a vocal but the exact same method hasbeen used)
2) highlight the start of the sample, (first word or a small section of the beggining of the sample) then select 'cut'
3) paste the short section into a new project or a space at the end of the current sample (you can delete it afterwrds)
4) process the section you have copied with whatever reverb plug in you use, set the reverb decay time as suited to how long you want the reverb to lead in later (you can allways adjust this with a volume envelope or fade later)
5) highlight the new part of the wave from the reverb processing just after the original section you used.
6) cut and paste into a new project or again empty space further down the original wave, now reverse then cut the new reversed verb section.
7) locate the very start of the orignal sample/vocal waveform
8) paste the verb tail at the beggining
www.emotioncomplex.co.uk/reverse_verb.mp3 and there you go :)