For décor/theme, eBay is very good. You can buy lots of mismatched tea sets/china quite cheaply, and other similar things, they look so lovely and very much in line with your theme. Unfortunately I don't have a link but there was an article or blog about a woman who did a vintage tea party on a shoestring budget, there were photos and it looked lovely, a lot of it was through eBay and a few simple craft projects. The big ticket items (dress, food, music, marquee) were haggled. She held it in her parents massive garden hence marquee instead of venue. Nonetheless marquees can be silly expensive, so I think it's equivalent to hiring a hall.
In terms of what you need, you really don't need much at all and it will still look great. Venue, some food, invites, clothes, flowers. A few extra decorative touches to polish it up, like little centre pieces, or swags of fabric. I agree that little potted plants look really nice - you can get some cheaply and put them in old vases or tea cups for an extra vintage touch.
Go to places: warehouses/wholesalers that sell to retail, markets, eBay, antique/charity shops and markets. You can get some simple fabrics cheaply in a nice colour to dress the hall, for example.
When we got married I wanted mixed country style bouquets and buttonholes, vintage-y, I spoke to our local florist with my overall idea and colours. She gave us a discount for getting the lot with her, did my bouquet and DH's buttonhole for free because we got bridesmaid/groomsmen bouquets/buttonholes, and it was cheaper because I asked for whatever would look good that was in season, rather than demanding an out of season flower. Florists have to get stuff at market very close to the day so if you're flexible you can get a great deal - what she created fit right in with the colour and style I requested, that was all that mattered and it looked lovely. My DGM picked her field flowers for their wedding reception! She lived in the countryside and it looked very nice (in pics, obv, I wasn't there!).
We made our own invites. I got some little additions on eBay - the bride I bought from had changed her colour scheme so was selling a lot of coordinating stuff and I got a good deal. There were quite a few listings like that.
I bought an antique knife for the cake cutting, just because I thought it would be nice! Again, a cheap eBay buy but a nice little touch.
If you want a professional photographer shop around, and have them for the shortest time possible. You most likely want pro pictures of the ceremony and perhaps afterwards with family. So have them for that time, rather than the entire day. Make sure everyone else has cameras (maybe appoint a few relatives who are alright with a camera) to make sure some pics get snapped. And take your own! My favourite photos from our day are of the ceremony/immediately after with family (professional), and then when DH and I went around to all the tables to catch up and armed with a camera. I wasn't keen on lots of posed photos though and going to special sites for pics.