September in BV is incredible. Days are typically warm and sunny, and it gets chilly at night.
BV is at ~8,000' (pretty high up there!). Please make sure to drink lots of water and electrolytes. Hydration is the best way to avoid altitude sickness. Those oxygen cannisters are a complete scam.
Yes! Peak season is usually at the end of September but it will be full-on fall in the high country. Coloradoans love leaf peeping so BV will be pretty busy.