there are a few of those websites...
danchoi
thiendia
kynu
gaigoihang
banhmy (just use for browsing, their system is bad, you have to pay for numbers, other sites have same girls don't pay for numbers)
and there are more -- just google for "gai goi saigon"
but if you go try, please come back and report about it, ok?
Start by just SMS the girl: "Em co ranh khong?" (are you free?)
Next she'll reply probably with hotel address.
Have Google Translate app ready on your phone in case she writes something else, but usually they don't, but that did happen to me once. I managed to fake my way through it by just ignoring what she said and asking where her hotel was.
KS means khach san, or hotel, you know that I guess.
So if she says something you don't understand, just ask "KS o dau?" (where is your hotel?)
Danchoi seems the most active to me.
Thiendia also big.
If you use Google Chrome it automatically translates the website for you. It will be a rough translation but good enough to figure things out. Expect to learn a bit with this aspect. The most important part you can handle: the photos and the phone numbers.
Also, keep in mind this is for locals, so sometimes the hotels look less than 4 star, if you know what I mean.
Most of the girls seem to be in the Le Hong Phong area of district 10 or else Truong Son area of district 8. But plenty in other areas too.
I can send you exact web addresses via PM but Google will surely put you on the right path too, faster than I can reply.
Good luck