The visa costs USD 20. You probably will have to go through a travel agent. The Vietnamese embassy in Phnom Penh issues visas for USD 55 within one week. Changing the entrance point on a visa should cost USD 10.
Some travel agencies do it faster.
Calle 18, No. 1802, 5ta Ave. Trade Mission, Dorotheenstr. In HongKong a visa valid for 30 days costs USD 50 and takes seven working days. Land crossings are possible, but cost more and the visa takes longer. Dawoodi Al-Mansour, No.