Vietnam Visa Question

Hi, sales,

My two  daughters and I have American passport. We will visit my mom in China at end of the year.  We are planning to go a cruise from Shenzhen to Hong  Kong to a port in Vietnam.  My question is if we three can get a tourist visa upon arrival?  I believe the cruise ship is only going to stay for couple of hours at the Vietnam port, then return to Hong Kong to  Shenzhen.  The entire cruise trip is 5 days 4 nights.

Thanks,

Sarah