i think most of the hotels 3 stars and below with charges USD$20-30 would close one eye and let u bring the girl in.
Some hotels would demand u to pay extra around USD$20 to cover up (extra room charge or something).
It is stricter at night.
You can try bring your key along in ur pocket when u go clubbing (just take the key out from ur kry ring and left the others in the room), because sometimes by the time u come back, the recep and the guard are always napping.....so u can tip toe with ur girl to ur room. lolx.
But like i said most 3 stars and below hotel would close one eye, but the girl need to handover her id/passport. And when the girl leaves, the recep would always call u which i think its a good security sense in case the girl ran off with your valuables.
I discover that more and more hotels are being built in a already spoilt for choices hotel zone around Cho Benh Thanh, so maybe next time the competition would be good for us.