just received visa, so will summarise for anyone doing this in future:
first need to log in to VFS and book appointment - this took me several days as i had many problems logging into their site (seems to have given problems for years). can only contact them via X/Twitter and their response time is not short.
appointments started after 1 week plus, so ours was 2 weeks ahead. i paid for a few VFS extras but probably a waste of money unless you're short of time. they don't take the EU visa fee yet.
so then need to fill in the visa application form, various languages accepted by Switzerland including English - all Thai paperwork needed English translation. i was named as "sponsor" so i provided statements/accommodation proof. flight & hotel details i got from "reservations" company named below.
appointment was over 1 hour late - seems to always be the case even if you pay extra to book a time slot. paid 80 euro visa fee. gave them masses of paperwork, and had to pay extra for even more paperwork they told us to include.
8 days later, received a phone call + email to say passport was on its way back to us.
they gave us a multi-trip visa as requested, valid for 1 month so covers the trip we specified and can now add another trip 2 or 3 weeks later.
best "reservations" company i found, reading Trustpilot reviews, was https://www.flightreservationforvisa.com and we had a few hiccups but all was completed in reasonable time. need to check the reservations when received, had problems with flight booking which needed help from their customer support. they also fixed my mistake of writing name wrong way round, and there's no charge for 1st change. the reservations expire after around 3 weeks.