It's a popular route as it's a once weekly seasonal service and fills quickly with the backpacker/adventure type crowd. The other added component is the stop at DEL on the return to VIE over most of the season. A lot of seats are taken with the demand on the DEL-VIE sector of the flight.

I'd suggest to the OP he looks for deals using TG via BKK. Not much of a backtrack and it's a daily year round service. Far more likely to get a consolidator type deal using that option - not to mention getting more miles for the account ! (Steer clear of the deep discounted options).