The vagina is a muscle so stretching it won't make it permanently loose. Neither will having a baby for that matter (I've had three and my gyno comments every visit on how tight and muscular my "area" is, not in a creepy way mind you, I asked about how to maintain my muscles there and he lets me know that I'm doing fine)...now having several babies in a short span of years will but if you take some time and rebuild your PC muscles you will tighten up again just fine. Just using a vibrator won't stretch you out appreciably mainly because it's the chemicals released during childbirth that cause the muscles of the area to stretch to their limit not the actual baby. Just keep up the kegels and you'll be fine!

Personally, I have a desire to keep those muscles strong because incontinence at a relatively young age runs in my family. The added benefit of powerful orgasm and pleasure for my lover(s) makes all the hard work worth it.