Question about letter validity

Hi,
My girlfriend and I are planning to visit Vietnam in January and we will ask for a letter for a 3 months entry. Can we ask for it now even though we only enter in the country in 3 months? Will the letter be still valid then? I know some official documents are only valid for a few weeks or months before they become obsolete so I need to check that it's not too early to start the process so we don't pay for nothing and have to pay again later.

Regards,

Jeremy Larrumbe