There are a number of buses that go from Phnom Penh to Ho Chi Minh City, I took Mekong Express but many prefer Sapaco. Mekong cost $13, took about 6 hours and offered WiFi on the bus up to the Cambodia border. They have a toilet on board and stop at a pretty decent restaurant with toilets for 20 minutes near Bavet. The land crossing was surprisingly painless. Even though I needed to get a visa on my way back, I wasn’t asked for a single bribe. You’ll need to get your Vietnam visa in advance, however (insider tip: it’s cheaper to use a travel agent than to go to the Vietnam Embassy in Phnom Penh).