How quickly can I reenter Vietnam (on a new visa) after leaving?

I am a US citizen in Vietnam on a 30-day visa I arranged via the Vietnamese eVisa site. I find that I'd like to stay in the country for longer than 30 days. I understand that I may be able to extend my existing visa for an additional 30 days through a visa agency such as yours and not have to leave the country. Can I also leave the country and immediately apply for a new eVisa? In other words, are there restrictions that would prevent be from being issued a new Vietnamese eVisa less than a week after leaving Vietnamn? Thanks for your help.