Trying to be as objective as possible.
To be fair to VNA they do offer a transition to bigger aircraft and they do offer a route from RHS to LHS, whilst not always a seemingly transparent process. You will pay for it$$, not just training costs, but no salary during training and reduced salary for a year and a bond..but not many contract airlines offer any sort of movement, so it could be seen as a plus..
Now for ATR Capts, Mig54 realize you are at the bottom of the list (though that doesn't stop some) if you are just joining, you will wait your turn..
Among the ATR guys who have gone up (320/321)a few have been successful and a few have failed, which results in a LOT of lost money and either an offer of 320 RHS (with additional costs for more training) or return to ATR (again with recurrency training costs to get back on the ATR) and no salary during this whole process 2-4 months.
Are you a young guy or an older guy, the older guys seemed to have had problems previously, whilst they might be experienced, the younger guys pick up the strict adherence to SOP and required Briefings a bit quicker, which seems to be a yardstick for how good a pilot you are, not your crosswind landing ability...
So if you come, come for the ATR job and assess where you fit in the scheme of things, VNA as I said to their credit do offer pathways to other A/C and seats, but those pathways for some are smooth and without incident whilst for others they are a costly, rocky path that leads right back where you started...