My Friend and I are volunteering in Cambodia for a couple months but will be traveling to Vietnam for our last month.
we will be arriving & departing from Pnom Penh & Im interested in road tripping from Ho Chi Minh city up to Hanoi.
firstly, can we get visas from Vietnam at the border? if so is it a bit of a nightmare and better to organise it prior. Secondly how long would it take form us on average to travel whilst enjoying the areas up to hanoi? can we train it back to prom penh in time without hassles in reg to being back in time for our departure flight?

There is no VOA at Vietnam land border crossings. You'll need to get your visa in Phnom Penh, about $60USD from the embassy, or from local travel agents. Try Capitol Tours, or Lucky-Lucky, they usually good prices/service.
if in Sianoukville get visa there very quick and easy direct from consulate but otherwise as above
Take note that there are reports that you can no longer get walk-in service (visa in 10-20 minutes) for $60USD in SihanoukVille. Regular service is reported to now be next day service, and faster express (extra $5-?$10)service is same day/walk-in? The reports are a little mixed, but it appears that the days of getting a visa in just a few minutes for $60USD are gone.
too good to last but in fairness they have provided an excellent service for a long while now
if only other embassys and consulates around the world had taken note that it was possible to process visas quickly and effeciently!!

You CANNOT get a Viet visa at the border unless you FLY from Phnom Penh or SIem Reap to Saigon.
No train going from Vietnam to PP !!!
There is a great way to go from PP to Vietnam : the fast boat PP-CHAU DOC, the Viet town border (boats leave PP at around 13h). Visit Chau doc + night, then the Mzekong delta Chau Doc-Long Xuyen-Sadec-Cai Be-Can Tho-Siagon in 4 days (buses and boats) ; 2 days Saigon ; NIGHT TRAIN Saigon Danang (long, so flight is better) ; 3-4 days Hoi An ; CAR Danang-Hue to visit on the way Marble Mountain and climb the superb Pass fo the Clouds (buses go through a tunnel), or train, which at least follows the coast all the way ; 2 days Hue ; NIGHT train Hue-NINH BINH + taxi NB-TAM COC and 2-3 days in the magic Halong-Bay-on-Land ; day bus Tam Coc-Hanoi (3 hours) ; 2 days Hanoi ; 2 days.1 night junk Halong Bay ; night of 2nd day, night train Hanoi-Lao Cai and 3 days in SAPA (1 day relax on arrival and 2 days trekking ; then you can take day bus to Bac Ha for the Saturday morning Can Cau Market and/or the Sunday morning one in Bac Ha, return to Lao Cai for night train to Hanoi.
