This is overcharging rather than scamming, but still….don’t book buses through agencies. Book yourself direct with the bus company or ask your hotel to phone them to reserve a seat for you (and pay when you get on the bus).
Examples from my recent visit – bus to Ben Tre from HCMC, quoted 190,000 VND by a city centre agency, paid 40,000 at the bus station.
Hoi An to Quy Nhon – quoted 250,000 VND (their starting price was 300,000!) actual cost 120,000.