Does a Myr Welder token created with Mimic Vat retain abilities added to the original?
No.

When an object refers to itself by name, it refers specifically to that object (regardless of its name). It does not refer to other objects with the same name. 

The Myr Welder card in exile never exiled anything.

The Myr Welder token on the battlefield never exiled anything.


  201.4. Text that refers to the object it’s on by name means just that particular object and not any other objects with that name, regardless of any name changes caused by game effects.