I am fully vaccinated American citizen and I have a flight scheduled to arrive in Bangkok July 8.
== UPDATE: June 7, 2021 ==
Yes, as at this transit will be available through Bangkok to Phuket. BUT you do neet to talk to your airline ASAP about this, since the transit would probably be heavily regulated because Phuket is a sandbox at the present time.
No one is absolutely sure as yet.
The situation on the ground changes from time to time. But the plan is to allow quarantine free travel to Phuket on July 1 and that has not changed as of this writing.
Here is the Latest News on this - dateline May 29.
Keep your fingers crossed and hope nothing changes!!