Editing with iMovie is the editing equivalent of shooting with an iPhone. You're severely limiting your amount of control. No, I do not recommend this. iMovie is designed for soccer moms, not serious editors.
In editing, precision is key. Every single frame matters. Every. Single. Frame. Try getting that kind of precision with iMovie. Your head will explode.
Vegas has two options for beginner editors, both of which are less than $100. I recommend Movie Studio Platinum ($95). It's a solid piece of software that will allow you plenty of space for growing. And here's the best part -- for the first 30 days, it's free! That's WAY more than enough time for you to both learn how to use it, and finish cutting your short film. Don't download it until you've reached post-production. In the end, your film will be better, you'll have had a much more positive experience in editing, and you won't have had to spend a penny on new software. You'll probably eventually make the purchase because once you experience the real thing, you won't want to look back.
F#*% iMovie! F#*& Movie Maker! These programs are created for people who don't pay attention to detail in their edits. These programs are created for video editing tourists. If you're local, if you actually give a damn about what you're doing, there are plenty of inexpensive options available to you. Sure, you can edit with iMovie, but you're asking for a major headache, for no good reason.