I would say 4 nights at the Beach and 3 nights in Bangkok. You can get a taxi or arrange for a car to take you from Bangkok airport to either Hua Hin or Cha Am. Someone may recommend Pattaya but I've never been tempted. Hua Hin or Cha Am would be fine for a few days, there are some lovely hotels at both places and the journey is about 3-3 1/2 hrs. You could do Bangkok first and then the beach or the other way round. Personally I would go to the beach first just to relax then head for Bangkok leaving you only a short journey to the airport on your last day. But if you have been spending a lot of time on the beach in Australia you may want to plan your stay with less time on the beach and more in Bangkok.
There are lots of day tours from both places if you want to see more of the city and countryside, your hotel reception would be able to book them. In Bangkok stay in a hotel near to a Skytrain station, the train is easy to use and it's airconditioned (useful in Bangkok's hot sticky climate) and you will be able to visit areas on your own if you don't want to take tours.
I can't remember any purely veggie restaurants , I'm sure there are some though I have never found any problems in finding veggie food in Thailand.

Judith

Thanks very much, Judith. Some good information.

Pattaya looks the most accessible place from Bangkok but from my limited knowledge and research it looks a bit too "full on", if you know what I mean! I'll look at Hua Hin and Chan Am.

Thanks

Maybe consider Koh Chang if you want more of a 'Thai beach' experience.

Note that anywhere accessible by road from Bangkok (Pattaya, Hua Hin, Pranburi, Cha Am) all get very busy at weekends - as they are so close to Bangkok! And if you don't want full on, avoid Pattaya at all costs!

By the time you've made it to your hotel in Hua Hin (3 hours by road away), you could though be in Koh Samui. One hour flight from Bangkok, and you can connect one way from your international flight and I would argue a lot nicer.