Presuming you have a bench since your asking advice about a vise.
A piccy or good description of your bench might help, as it might help us advise you if its suitable as is.
For instance, I think its a very good idea to have a trusty reference surface to plane on.
You will be able to lay your timber on the surface and see which way your timber is bowed.
Use a light to shine underneath, or rub the bench with a crayon and rub your work against this to show you the high spots
the only points of contact to be removed.
Or hold one end of the length and see if it pivots from the end or spins like a propeller.
If it spins like a propeller ...its high in the middle
Sit your timber on the bench, hollowed side down, as the timber sits on the edges and doesn't see saw while planing.
This means that you will be 95% scooping the timber only from the middle on the other top side.
Always aim for the slightest hollow on both sides of your work... a paper thickness
it sits correctly then on the bench.
Don't dive the plane off the edges on this top side ..leave about 5mm of each end ...
Finish up with a finer cut with one or two at most cuts.
Another 51/2 plane would be better for the work though so you could leave the 4 for smoothing
If you can have a reference surface it will be a help ....you might choose to lay this on top of your bench,
In which case your wooden vice jaws will need to be taller .
You will enjoy planing that meranti, its lovely stuff to work