Does anyone have any recommendations on where to get tickets for this?
On Manthaturat road in Vientiane, you will find many agents that sell the bus ticket Vientiane - Hanoi.

UNless this is avery recent innovation, and if you mean the tipical Chinese sleeperbuses (htat ply to KunMing ex VTE), there is only the 'hard seat' (read cramped seat) toyota minibus doing that route, takes like 20-24 hrs, depending on road/wheather and thus includes a non-sleepable overnight on very, very rough roads.

It's an OVERNIGHT bus but not a sleeper bus and tickets are sold from Luang Prabang to Pakse from travel agencies. Don't expect anything like the VIP buses that ply southern Laos or all of Thailand.
I just did the Vientiane - Hanoi bus trip a week ago. Paid USD20 for the ticket. You can get it for USD18 if you shop around, but I couldn't be bothered.
Details of the trip:
5.30pm - picked up from guesthouse by minibus and brought to bus station
7pm - bus departed for Hanoi. No aircon or toilet onboard. Bus was half empty, so all foreigners managed to get 2 seats to themselves. Some travellers who bought tickets for Hue and Danang were surprised to learn they had to take the same bus.
9pm - stopped for dinner in Paksan
2am - stopped an hour away from border to wait for check-point to open. Weather was quite chilly.
7am - reached border. Vietnamese immigration demanded 2000 kip for entry form and USD1 for processing the passports. This was for all passengers.
11am - reached Vinh. The travellers heading to Hue and Danang were dumped at a roadside cafe and told to wait 4 hours for a connecting bus.
5pm - arrived in Hanoi.
Overall the trip wasn't as bad as I expected.