one of the great things about having a daw is the copy and paste thing.... copy the vocal to a new track... take the newly copied vocal and compress it really hard, like 8db or more, with a nice sounding compressor, something with some thick character is best rather than a really clean one... after scrunching it really good, add some eqing to exite it and make it stand out, typically around the mids and highs, but use a parametric to sweep through it and find what areas thicken it up. You may want to try rolling off the bottom, you may not... depends on the source. After that, try using something to dirty it up, like the PSP vintage warmer (free demo) or maybe an amp-sim plug-in, get it all excited with harmonics and EQ and have it squashed all to hell, then bring that track in under the original vocal (which is still your main vocal and shouldnt be compressed much at all so as to maintain some amount of dynamics). If you use an amp sim or something, you may have to use a high pass to keep the low end from getting muddy. Eq the original track as needed, but as little as possible. Roll off extreme lows and compress about 1 or db, as needed.
Try it, let me know how it works out. You most likely will like it. Just remember to keep the new track kind of hidden behind the original.