In my experience, guitar-oriented distortion/overdrive pedals are not good for adding more subtle "vintage amp" type sounds. They produce lots of high frequency overtones. As they're designed to be put through a guitar amp, it's expected by the designers that these overtones will be heavily smoothed out by the amp's loudspeakers to produce the finished sound. This simply won't work with studio monitors or powered PA speakers, they're too hi-fi. Also the inputs are designed for very low level guitar signals, the output of any modern keyboard (even a Nord

) is simply far too great.
One thing that might be worth trying could be an guitar amp simulator of some description, although that would not really be a cheap option. I have a couple of Sansamp Character series analog amp simulators, a Vox AC30 and a Fender combo. If I get a chance in the next few days, I'll dig them out and see what they sound like with some EPs and the Clav.
Gustavo wrote:But what I did found usefull was a small delay, the memory Toy. Now some guitarrists complain about it not being so good, etc. But I found that if you use it and just apply a little bit of delay and let your signal go through it you will get a more "vintage" sound compared to the more "crispy" and modern sound you have.
Using an old "lo-fi" delay line is a great way of giving character to sounds. You can also use one to simulate reverb on recordings as you can add a lot of it while not cluttering up your mix as much as a modern reverb. It's particularly useful on electric guitars but works with vintage synth and keyboards sounds as well. I also once had a small spring reverb unit which was just amazing. Rubbish on drums, but luscious on pads and analog leads. For DAW users, there are surely a ton of plugins that would emulate these vintage kinds of devices, as well as choruses, phasers, flangers and whatever else comes to mind.