Getting to Vung Tau from SGN
 

I am landing at SGN late at night and need to get to Vung Tau. I guess I'll be spending one night in Ho Chi Minh city, but if possible I'd like to avoid that.

I've seen taxi/car services run at ~ USD 100 each way ?

I've also seen a few bus services, but it's quite hard to get much online (https://futabus.vn/, and the jetstar bus at http://www.jetstar.com/vn/vi/what-we-offer/bus-service).

Would be glad to know if there's any better source for information about bus services to Vung Tau...

Was at Vung Tau in 2014. Trip was via minivan and price per ticket was appx $110K VND, or $4 which is 25 times cheaper than taxi.

Check at wikivoyage article here - https://en.wikivoyage.org/wiki/Vung_Tau#By_bus

Through Expressway: the fastest and most comfortable ride which takes about one and half hour. start from Ben Thanh market, get through the Thu Thiem river tunnel, than take the HCMC-Long Thành-Dầu Giây Expressway before connecting to Route 51 in Long Thành Town. It is 30 km shorter than option 2 and the view of the remote countryside is very gigantic. read more" http://wikitravel.org/en/Vung_Tau

From SGN, you go to Mien Dong coach station to catch Hoa Mai, Thien Phu, Rang Dong, Phuong Trang, Mai Linh buses to Vung Tau with the price of 60,000 - 80,000 VND (3-4USD).

Don't attempt that drive at night! There's also hydrofoil service:

http://vinaexpress.com.vn/ (click the UK flag for English)

You can refer Hoa Mai bus to take a bus with 3-4 USD. I would find a web vexere.com, you can book a ticket online through this web.