If booking on the Thai Airways website, first select origin, destination and dates, then you will go to the price/date matrix select as appropriate. Step 3 you will be asked to select flights and a fare type. A list should appear with Thai Saver, Thai Advantage, Thai Saver Plus, Thai Flexi Saver and Thai Flexi Plus. Each is specified with "no mileage accrual" or "mileage accrual". Select ones with Mileage Accrual and you will not be in V class.
Do you have dates?