Dear Joseph L,
Thanks for your attention on our website.
The validity period of your e-Visa begins from the date specified in your approved e-Visa document. You can enter Vietnam on any date within this validity period. But you must leave by the Exit date specified on your E-Visa
You cannot enter Vietnam before or after the validity period specified in your e-Visa. if you want to enter Vietnam before or after the validity period, you have to apply for the new E-Visa.
Or you can enter Vietnam for 15 days on Visa exemption with your valid UK passport.