If you have booked your ticket directly from Vietnam Airlines you can check here: https://fly.vietnamairlines.com/dx/V...1&locale=en-US

If you have booked your ticket at a travel agency or on an online booking site you can’t select a seat on the VN-Website. You should contact the customer service where you have booked the ticket ... or wait until online check-in is open (24h before departure time).