How can I edit all the files returned by find in vi in Linux?

This should do the trick:

find . -name "*.txt" -exec vim {} + 

Use Vim, it's better for your health. :-)

The oft-overlooked + option to -exec makes all filenames (up to line length limits) appear in one line, i.e. you still get all the files opened in one vim session (navigated with :n for next and :N for previous file).

With vim -p you get a file tab for each file. Check :help tab-page-commands for more details.

With vim -o you will get horizontally split windows for each file, vim -O vertically split windows. Check :help window-move-cursor for more details.

Note that the previous version of this answer, vim $(find . -name "*.txt"), does not work with spaces in filenames, and has security implications.

Piping into xargs vi gives a Warning: Input is not from a terminal, plus a terminal with completely bogus behaviour afterwards. User grawity explained why in a comment below, and with a bit more explanation in this question.

Or run vim and from there:

:args **/*.txt