@Ashlie *******
No set waiting time..
However I don't believe you can apply online for another visa while still currently in Vietnam..
You have to exit and then apply online and wait for the processing times... usually around 4 to 5 working days ..
Unless you speak to a visa agent and get some urgent processing done which does cost extra money..
I suggest you contact Emily Visa (She has a profile on Facebook)and ask her about options..
She is a reliable and trustworthy visa agent in Vietnam.