No, not at all. When choosing a vinyl window, be sure to choose quality and do not base your decision on price or you will wind up wanting to do the job twice and that can get very expensive. Some of the high quality brands most pro's recommend are Okna, sunrise, gorell, and soft lite. These are all well designed and more energy efficient than any wood window.
Stay away from the the lower quality windows such as Xact, silver line/ American craftsman, and pella vinyl.

It's difficult to say for sure. Most vinyl windows have a frame depth of 3.25" whereas many aluminum windows have a thinner frame depth. It also depends on the opening and how your existing blinds are hung. If you have a drywall return, then you may have to move the brackets inward slightly to accomodate the deeper frame on a vinyl window. In most cases, the blinds can be re-used. I would ask the window professional you decide to deal with. They should be able to answer your question after they have seen the set-up.

That is true with the Sheetrock return , however blinds usually are not an issue.

Ok, good. It doesn't sound like I made a huge mistake. Our frame depth is around 5" if you include the window itself. These are picture windows so there won't be any extra space taken up by opening them.