Every time I've done this with extract has been with partial mashes, which are easy and I recommend them. Just make sure you have excellent sanitation, ferment below 70f, and hit your gravities. By hitting gravities, I mean you should shoot for an OG of 1.045 or above.
For partial mash, you just load up a strainer bag with the Vienna, Crystal malt, and maybe a few pounds of base malt replacing some of the extract (see death brewer's brew in a bag thread). Go with 1.1 quarts per pound, and hold the mash at the temperature Edwort calls for, for about 1 hour. Vienna needs to be mashed anyways, so why not throw in a few pounds of base malt to save money and add flavor?
The last extract beer I made was a partial mash Haus Pale, but with only 2 pounds of extract, the rest was grain. It was maybe my 3rd best beer so far.
