@Marek *********
back before the switch to 60-day exempt, I used to get a tourist visa, do a border run to Vietnam, and then come back to Thailand for the 30-day exempt (and then leave for Indonesia and come back for another 30 days a month later). So, I think you'd probably be fine. The one concern I'd have is that people have been reporting challenges when they try to enter repeatedly with an exemption now, especially if they've done extensions in the past.
Personally, I decided to just do a multi-entry tourist visa for my upcoming trip, so I won't have to stress over it. The consulate in HCMC took 2 business days to get back to me with a request for a newer photo and an itinerary (I had only made reservations for my first 60 days of flights and hotels), and then took another two business days to approve me.
So, if you're just doing one exempt entry, I doubt you'll have any issues (but I'd defer to the experts). If you're planning multiple visits after, I think you can take the anxiety out of the process if you do a border run to HCMC, apply for a tourist visa, and include a proposed itinerary in the PDF of your plane & hotel reservations.