In Virginia, is it okay for the landlord's agents to personally serve a 5-day notice to pay rent or quit? Which is proper or improper service: a personal statement certifying proof of service? Registered mail? Sheriff posting notice on the door? All or none of the above?

This would be your personal notice, prior to going and filing with the housing court? Nromally registered mail is used, Since you can expect them not to move and this is merely a requirement prior to filing in court. Have you asked the clerk of the court what they require as proof ( if any)

Frankly I think you are nit-picking. For the landlord to serve a pay or vacate notice, you have to be behind in your rent. Whether the service was proper or not, you apparently have not paid your rent. So why not pay it or vacate?