I am traveling from Vientiane to Bangkok (Lao airlines) and then Bangkok to Kolkata (Indigo). So it's 2 separate PNRs.
The layover is 7 hours. Would I need to apply for a transit visa?
Indian citizens are currently eligible for visa-free entry into Thailand.
I have done similar transits (Laos to BKK, BKK to either HKG or SIN), with or without check-in luggage, without issue. Having two PNRs didn't matter: the airlines were happy to send me on my way, and when I had luggage, because they interlined (even when they weren't in the same alliance), they would tag my luggage with the flight numbers all the way through. One of the great conveniences of flying in Asia...
The problem here is mostly luggage, as these two airlines probably don't interline, Indigo being an LCC (and Laos airline being... well whatever it is). So you will probably have to pick up your luggage and go upstairs to check in again.
A transit visa wouldn't help, as it would be only to stay airside. But the good news is, as mentioned in the previous answer, that you can get visa-free entrance right now. And 7 hours is plenty to do the whole Immigration, luggage, Customs, check-in, security thing.