Does anyone use a USA mail forwarding company

See if there is a UPS Store in your desired area.  These are contractors and frequently have mailboxes and will forward mail, fpr a fee of course.  One nice thing is that they provide a physical address as opposed to a PO Box for certain entities that demand such.  To avoid having to pick up your check at the consulate or Embassy you can't let them know you are in Vietnam.  The downside is it will be about $50 each time they forward your mail or parcel in a UPS envelope or box.  Even Registered, don't place any faith in anything involving the VNPT arriving on time.  Even their name, Viet Nam Postal & Telegraph, evokes an older, slow moving era.

Thank you THIGV
That is the way I will go. For SS and the VA they only send a couple of letters a year but having a US address will ensure I will get the letters in a timely manner. Yesterday I received a letter from the VA dated 5 December 2016 and that was with the help of VA Manila who mails it to the US Consulate HCMC in care of me who then forward it by registered mail in country to me.
On another note I never had to pick up my SS check at the Consulate. It goes Direct Deposit to my US bank. Additionally, I do not have to check in any longer with the Consulate to prove my existence.