Newb1,
I wouldn't discourage you from filing an application for a K1 visa after visiting for only a week. This was in fact my own intentions, but a divorce gone haywire kept me from doing it. Heck, you did know her for a year online before taking that trip, which is far more than I did. But enough about me.
While I wouldn't discourage you from filing for the visa- and as long as you have photographs from your trip showing you've met you will probably be approved (assuming there isn't a bunch of other stuff I don't know about), I would highly encourage you to make at least one more journey over there while you are waiting for the interview. It's not a mandatory thing, but it will look much better in the eyes of the consulate to show you have an ongoing relationship. Since you are new to the process I will make it clear that having your K1 application approved by the USCIS (NOA2) is not the same as being approved for the visa itself- this part will be done by the HCMC consulate and their standards are MUCH tougher than the USCIS.
You are engaged- so assuming you've already had the engagement ceremony, you should begin the process. Heck, they may ask why you got engaged in January but didn't file for a fiance visa until June if you waited for a second visit. That would be kind of wierd, right? "I was sure I wanted to marry her, but I wasn't sure I wanted her to be with me just yet"

This is something you say after you've been married for a while. "I was sure I wanted to revisit Vietnam with her, I just wasn't sure I wanted her to come back with me"
Try very hard to make it over there at least one more time though. You won't regret it.