There are three ways you find your VietTESOL Membership ID by yourself.
When you sign up for VietTESOL Membership, upon your submission of the registration form, you will see your Membership ID in red color as in the screenshot below:
Check your email inbox for an email from VietTESOL Association with your membership details, including your VietTESO Member ID.
Go to VietTESOL Association's official website at http://www.viettesol.org and login with your username and password. If you forgot your username or password, you can retrieve either or both of them using your email address.
Once you have logged in, go to the Membership section and choose My Membership. You will see your Membership Profile with your Membership ID.