Converting MP3 files to small size

Is it ok to convert a MP3 file from 128kbps to 80kbps vbr? I mean, will there be a difference if I convert 1411kbps WAV to 80kbps MP3 vbr? How much will be that difference?

When converting from lossy format to lossy format (i.e. reencoding from 128Kbps to 80Kbps) you will have a difference. You already are starting with a file where data has been removed, and you are removing still more data.

If you convert from a lossless format (e.g. WAV) to a lossy one (MP3), you will also lose data (indeed, this is the entire point of lossy compression: removing unneeded data in order to get a reasonable file size).

As for the differences, often they are subjective: some people can't tell the difference between lossless files and 128Kbps files, some people can. You will have to test them experimentally and find which parameters give you a good quality/file size proportion.