What pool calculator did you use? The one I use is here: http://pool9.net/bleach-calc/
There is another one that some people use but I don't really care for that one. I cannot answer why the big discrepancy. It could be that your volume is not really 40K or it could be because of some other reason. I don't know.

Can you give a full set of readings taken this morning, please. Is the pool cloudy?

It is probably ok to vacuum this morning unless the chlorine is still ridiculously high.

High chlorine can bleach out a liner. It doesn't always but there is no way to know what point is 'too high' for various liners. They are all different.

Skin can take pretty high chlorine levels, but swimwear not so much. So, if you do swim, wear an old suit. Let's wait to see what your readings are first.