You can apply straight away, not need to wait til Monday.
Get your online application done before you arrive and save it. Then just add the proof of location PDF when you get in the country and gather the evidence.
Ensure to double check all fields, especially passport info before submitting. Some info in various fields changed after saving my application and if you get something wrong, your application is cancelled.
Booking reservation for 2 weeks is fine. Should only take 7-10 days max (mine 4 days in June). Mention in your cover letter your travel plans in Vietnam, you have a 2 week reservation booked, may travel etc. Ensure to apply for a 4 week Vietnam visa (minimum), in case you need to stay in Vietnam if things take longer. The dates you put for your Vietnam visit is what they give you for the visa.