Can we apply for Hongkong travel visa with Vietnamese passport holder?

You can apply for HK entry permit from there.
The first step is to prepare the required documents including copy of your passport, the filled-out application form attached with a recently-taken passport-sized color photo, your residence permit in Malaysia, proof of your financial support, transportation arrangement, hotel reservation and your travel itinerary.
And then submit your application to the embassy of PRC in Malaysia to apply for it.

Thank you for your information. However, there is one more thing about the financial support. How much money I need at least in my account for financial support?

Vinh, there is no minimum amount for the money in your account, but based on my experience, the more, the better.

Hi George, Can I ask one more thing. We already booked flight ticket from 21th May to 25th May 2016. How long it will take for visa application process and when we should apply for this?

If the China embassy accepts your application and handle it by itself, it only takes four working days. If it transfers your application to HK, it will take about one month. So you should apply as soon as possible.