Anonymous participant 474 Laos doesn't require anything. It's the Thai side that requires it. And normally that's only for people trying to leave and return visa exempt (without a visa). If you have a visa, like DTV, you should be fine to bounce out and back immediately.
Sponsor letter is only for dependants applying for a DTV based on their spouse or parent. The holder of that sponsoring DTV can also sponsor the dependants so they don't have to show their own bank account.
Many embassies ask for the 500k as well as proof of payslips so they know you can support yourself while you're in Thailand.