I have two video files.

Video A is complete and runs at the correct speed, but is VHS quality.

Video B is incomplete and runs too fast, but is full 1080p HD quality.

I want to merge them into Video C. Video B needs to be slowed down to Video A's speed, and where Video B is missing parts, I will use frames from Video A instead. Video C will run at the same speed as Video A, and I think I'll just use the whole sound track from Video A even though it's lesser quality.

To be specific, Video A is encoded at 29.97fps, but every third frame is an interlaced blend of the frame before it and after it, so it's more like 19.98fps. Video B is encoded at 23.976fps, full progressive and no interlacing, but this is too fast.

What software should I use to merge these videos into one, in the manner described? Free if possible. I've toyed around with VSDC and Shotcut. Frame precision is very important. I think I need to slow down the HD footage to exactly 83.33% of its original speed. The total number of frames will stay the same, but the duration of each frame will be longer, the fps will be smaller, and the runtime will increase. VSDC doesn't seem to let me change the speed that precisely; I can only slow it down to a multiple of 1%, unless I'm missing something.

Also, what frame rate should the output be, if I want to burn it onto Blu-Ray and maximize quality? 1080p @ 19.98fps would be ideal, but I don't think Blu-Ray supports that. If I triple every frame, I'll get 1080p @ 59.94, judder free, but I don't think Blu-Ray supports that either; I'll need to downgrade to 1080i or 720p to make 59.94fps work.