The command should be in one line with no breaks.

It's a combination of two commands: the grep lists all the files that match the pattern (regular expression) and hands them over to the sed, that does the find & replace part.

If the grep doesn't find any files, it won't work. The pattern I used is:
1. a colon (
2. a plus-sign (+)
3. either 8 or 9
4. seven digits (from 0 to 9)
5. end of line ($)

Maybe you find a pattern that works or you give me an anonymised vcf-file for testing purposes.