You want a video button, but don’t want it to link to your timeline? Then don’t use Encore’s button video thumbnail layers at all.
Instead, create a facsimile of it in AE or Premiere and use that as a video background for your menu, and link your button to where you want it to go. Since you don’t use a button video thumbnail layer, nothing will change when you link to a menu, since it’s all fixed in the video background layer for the menu.