No you usually don't have to include a flight ticket for single entry tourist visa
Depends on your country's consulate but here's UK, only the multiple entry requires it. Single is just completed application form + photo + the fee
Flight ticket may come up as an official requirement if you Google it and land on some old outdated embassy websites, but so does 'proof of 20k baht in funds', that doesn't get asked for either at the time of application
Even if Scandi consulates are stricter since you haven't ever been to Thailand they likely won't enforce a flight ticket because you haven't been to Thailand so you don't look like an illegal worker or anything
And don't get that multiple entry visa cos you have to leave after 90 days anyway, just get two singles, one by mail one in Vietnam when you go
If you really need one though Bestonwardticket.com is better than flyonward.com, cheaper and lasts 48hrs not 24h