Note: This was written up in response to an email inquiry... we thought we would share it here, since many of you may have the same questions!
Different brands of vellum are different levels of ph /acidity. For scrapbooking, the thinner the vellum the better the ph for archival purposes. Don't use really thick vellum meant for card making or gift packaging. Look for thinner stuff.
Generally, vellum is not a very absorbent surface. But you can work around that. Different brands of vellum have varying 'finishes' on them. This finish is a coating like wax almost that makes the vellum smooth. The problem is it is also often almost waterproof! The culprit for your smearing is this same waxy surface. So check different brands. What works with my printer is DOTS and Stamping Up brands of vellum. Other brands work well too but I just happen to have some of this so I use it most often. I have an HP deskjet 694c. Experiment a little to see if some brands other than the one you are now using will absorb the ink faster for you.
If you want to try another option on the vellum you have on hand, try lightly going over it for about 3 or 4 minutes with a crumpled up paper sack as if the sack was sandpaper. Do this before you send the vellum through the printer. This sometimes breaks down the coating on the surface just enough to allow ink to penetrate and dry.
Lastly , on most printers you can choose Good Better or Best quality copies. Be sure to set your ink quality for "economy" or poor printing . This may seem strange, but you really don't need much ink on the surface of the vellum. A very little ink will go a long way and we want it to dry before the next line is printed....so it won't smear. So choose the poorer setting on the choices. Also when printing journaling or wording, use a 'thin' font and not a thick one for the same reasons. It will take some experimentation, but using your printer with vellum will work! Good luck! I hope this helps!
