I removed ceramic tile and high spots of thin set mortar. Can I now install 1/4" plywood and install vinyl plank flooring?
The answer depends on how much thin set is remaining, specifically how high are the remaining thinset patches.
The general rule I have used is; If a bump or dip is smaller than the thickness of a dime it is ok. You can lay the LVP over it. (This is assuming you are using a floating floor and not glue down.)
If the thin set remaining is as described above there is no need for plywood.
If the lumps of thinset are thicker, you would be better off spending the time scraping or grinding the high spots down and filling the big dips, rather than adding plywood. Should the surface still be a bit too rutty, you can add a soft underlayment. There are many types to choose from.
Additionally, It is too late now, but LVP goes over tile very well.
LVP and LVT are very forgiving and many DIYer fret more than necessary over the condition of the floor it is going on.
Finally, you may have areas where you hear a click or crunch when walking over the finished floor. Give the floor time to relax ( a few days to a couple weeks) and the noises will go away.