There are several Vintage Clothing communities online that you can join, or visit to glean info on vintage. Each one offers different information and environments.
Vintage Clothing & Accessories forum on ebay - http://forums.ebay.com/db1/forum.jspa?forumID=92&start=0
Most of the participants on this messageboard are ebay sellers. Here people post pictures or questions about their latest finds. The regulars there know their stuff and are quick to reply with a friendly response. You don't have to be an ebay user to read the posts, but you do need to register to reply or to make posts yourself. You don't have to be a seller to ask questions here either, anyone is welcome to reply with helpful info or to post questions about your own items. This is probably the most active messageboard on vintage that I have seen. I check it out 4 or 5 times a day to see what's going on. I usually lurk, but I reply occasionally when I have some info to add. The messageboard is a friendly community with only an occasion spat or the passing troll, just ignore that.
The Vintage Fashion Guild - http://www.vintagefashionguild.org/
Founded by a group of vintage clothing sellers who combined their knowledge to create this online resource. It provides a timeline, history of hats, shoes and hairstyles among other things. Also a guide to help indentify furs and ethnic dress. A very useful resource that they offer is the 'Label Resource' with a collection of images of designer labels, the eras that those labels were used, and brief designer history. Their online resources are available to everyone, no need to join. You do need to sign up for the basic membership to view and participate on their messageboard. They also have a paid membership with benefits for vintage clothing sellers, membership is by approval. I am not a paid member. I did sign up for the basic membership and I have checked out the board a few times.
Fashiontrac - http://www.fashiontrac.com/
Another group of vintage clothing lovers and sellers formed this group to provide fashion information. It has a seasonal newsletter with fashion trends, vintage events, auctionhouse vintage sales, featured sellers and a public messageboard. It's past newsletters are archived online, so there's some good info to check out there. Each season they have an indepth look at the fashion shows and report on the upcoming trends. Membership is free with no signing up at all, unless you'd like to receive their next newsletter by email.
Here are 2 Yahoo! vintage communities that I just joined - I'm brand new to both, so I can't comment on them very much. They both look to provide a nice, friendly helpful environment for vintage lovers.
Corsets & Crinolines - http://groups.yahoo.com/group/corsetsandcrinolines/
This club is about ANTIQUE VINTAGE CLOTHING, HISTORICAL RE-ENACTING, HISTORICAL COSTUME MAKING/SEWING and HISTORICAL CORSETRY! It is a private group, membership pending approval.
Art Deco Living - http://groups.yahoo.com/group/vintagedeco/
Vintage clothing and accoutrements of the Art Deco era
If you know of any other online vintage communities, please let me know about them, I'd love to check them out.