My experience with the Vecturo and plunge base is that you will get deflection particularly along a long cut. And particularly on thicker materials. 7/8" is probably pushing what the Vecturo can accurately cut. I've tried it with 1" material and did not find the cut to be 'furniture quality' along the cut and depth. You might try one of the round blades with the blade plunging against a 3/4" piece of wood along the cut as an option to give you both more width along the cut and a flatter surface to reduce deflection.
My personal choice would be the TS55. It will give you an accurate cut with zero deflection. You will need to finish up the ends, but that could be done with a Vecturo or a jigsaw or even a handsaw. Might need some sanding or a rasp to finish it off.
You will very likely get deflection with a 1/8" router bit, most noticeable as you go deeper into the cut. From the collet to the end you may have as much as 1.5 to 2" of distance on the bit, which I expect would likely deflect. Plus a 1/8" bit won't give you a quality cut if it's meant to cut 7/8" deep, even with multiple passes. With a 1/8" bit less the depth of the flutes, you don't have a lot of metal.
What is the size of the opening you are cutting?
