each office has different requirements. Mine wants things like new photos of wife and family in front of house, kitchen, lounge and bedroom each year. An updated marriage certificate verification letter less than 2 months old and more.
On the Vientiane Thai embassy site, it asks for this additional letter ONLY if you were previously issued the same type of visa.
"If the applicant was previously issued a Volunteer Non-Immigrant O visa, he/she must also present a certification letter for a foreign volunteer issued by a relevant government agency."