There's a couple things to flesh out:
1) Vegas is primarily a video editor, are you using film, or just making animations?
2) Will this be a consistent program you will use in the future?
3) Do you want an FLV specifically or simply making a video in flash. You can always convert a video file to FLV
Flash is not hard to learn, but it takes a while to get good at making videos in it. Unless you use scripts, videos will take you significantly longer (since you operate frame-by-frame otherwise), and you'll have less impressive effects.
If you're serious about making videos in the long run, then get Flash CS3/4 (I haven't used CS4 yet, only CS3) and start messing around/reading tutorials etc. If this is for a video or two, then stick with Vegas/AfterEffects/Final Cut