Vellum was all the rage a few years ago and it's making a comeback for good reason. It's versatile and fun to use in a scrapbook layout. Vellum is one of the few embellishments that you can use in almost layout without all of your layouts looking the same. You can do so many different things with vellum to change the look and feel of the page.
All of my scrapbook pics are on Flickr and I had big plans of reuploading the pics here. Apparently you can't save a photo from Flickr (it saves as 1x1 pixel) so no photos. If you're curious, you can see my scrapbook set on Flickr here.
I love to tear vellum. It tears so well and gives a new look and texture to the page. When you tear it, tear really slowly to make sure you don't tear into the picture or words. You can tear even a small stripe from vellum. You can see torn vellum here.
Vellum holds ink well. I love to ink the edges either on a cut piece or a torn piece. It really makes it pop on the page and works really well if you're attaching it to a light background. An example is here.
Vellum is great to draw on as well. To keep the muted look, flip it over and use markers on the back side. You can make a really custom embellishment to match your page perfectly. Examples are here and here. On Fabulous Fall I colored in the back side of the leaves and the page title to make it match my layout. On the fishing page the sign post is a piece of vellum I used Creative Memories markers to color.
Think outside the box. Just because a quote is laid out nice and centered on a page doesn't mean you have to keep it that way. If I can't fit the perfect quote on my page I cut it up. I cut up a quote on this page and centered it myself to get the effect I wanted. On this page I used the cut up quote as a border for the whole page.
I used vellum words to make tags on this page. The patterned paper still peeks through to coordinate with the layout and I got the words I needed.
Attaching vellum can be difficult. I like the Creative Memories vellum squares. The disappear completely with CM vellum, but do show a bit under other vellum. I've heard you can run vellum through a Xyron but I've never tried it. I also like to attach vellum with brads and eyelets. I don't have any pics of these layouts though.
Have fun with your vellum! I hope I taught you some new techniques.