First of all, you need you set up your node setup to look like this:
https://imgur.com/wRC0O8E
now, adding the vertex colors to the texture:
then you add a new image texture node, press the button labeled "+ New", set the name and resolution (resolution should match original texture resolution), make sure it is highlighted yellow.
then on the properties thing (on the bottom-right), click the TV icon, change the render engine to cycles (if it isn't already)
open the bake tab, change the bake type to diffuse, uncheck Direct and Indirect under Contributions and press the Bake button.
once it's finished change the base color to the new texture
https://imgur.com/wRC0O8E
now, adding the vertex colors to the texture:
then you add a new image texture node, press the button labeled "+ New", set the name and resolution (resolution should match original texture resolution), make sure it is highlighted yellow.
then on the properties thing (on the bottom-right), click the TV icon, change the render engine to cycles (if it isn't already)
open the bake tab, change the bake type to diffuse, uncheck Direct and Indirect under Contributions and press the Bake button.
once it's finished change the base color to the new texture