How to combine and negate these two patterns together?

You can use :v to execute a command on lines that do not match a pattern.

This requires that you use a single pattern though... Which in your case you can easily do by using the \= modifier to optionally match the last item.

So to delete all lines with either 2 or 3 characters, you can use:

:g/^...\=$/d

And to delete all lines except those with either 2 or 3 characters:

:v/^...\=$/d

Following would be my regex of choice

:v/\v^.{2,3}$/d

Try it online!

Options: Case insensitive

• Assert position at the beginning of the string ^
• Match any single character .{2,3}
  • Between 2 and 3 times, as many times as possible, giving back as needed (greedy) {2,3}
• Assert position at the very end of the string $

How about "delete all lines with less than two or more than three characters"?

:g/^.\{,1}$\|^.\{4,}/d