How to get rid of the invitation of street vendors in Vietnam?

Travelling to Vietnam we usually meet the local people with some books or some kind of food on their hand invite tourist to buy some thing for them. They always walk along with us and say "buy some thing" or "buy for me".

We are tourist and we want to treat them politely, normally we say "No, thank you" but they still keep walking with us and try invite to buy some thing, some time they knelt down and begged for buying. In that situation, we have to buy some thing for them although, in out mine don't want to buy any thing, and don't have any needs.

So what could we do when we meet the street vendors?

To me, many time I travel to Vietnam, so I have experience to treat them politely but do not have to buy any thing for them. When you meet any street vendors at any where, what ever they invite you, just keep silent, do not say any word to them, do not pay attention to what they are doing. It mean you do not care what they are doing, Keep walking. Be sure that, in few minutes they go away.