As soon as i left my hotel, i was approach by many, many viet guys on motorbikes (Bikermen), offering to take me to massage, women, drinks... etc especially, if you are walking by yourself. Some of them don't even have bikes.
You can make your own judgments, but here is how it works (I think):
They try to make friends with you, and offer ride to where ever you want to go around the city. They get commission for taking you to a place for you to spend money. They can take you around the city whole day, and even wait for you. But at the end of the day, they ask you for $50 US for the day, regardless of what you have negotiated in the beginning.
My view is that, they are essentially ripping you off before you are a foreigner. I wouldn't do any deals with them, because their sole purpose is to get as much money from you as possible. They can succeed because they are good at making you trust them.
They only deal i will do with them are short trips, for example: Get from A to B for 30 VND (2 AUD), take it or leave it. But then again, it takes effort for you because they will try to sell you more things etc.
I rather take a Legitimate taxi with a meter. less questions, less hassle.
In my case, i did hire one of these guys for a day because i negotiated with him 200k (14 AUD) for the day. But, of course, at the end of the day, he wanted more, and i said PISS OFF (in the nicest possible way), trust me, its not worth the emotional effort.