shower or tub only? Tub-shower combination?
The most crucial information is missing, Jim.
If shower water is going to get on the tiles, skip NO steps; you'll need the vapor barrier. And leave a space at the bottom of the 1/4" Hardibacker so water on the tub itself doesn't have any contact point to it. There is always some way for moisture to get behind tub caulk, and Hardiboard does let moisture inside it, and this will wick...
If this is a tub-only, you have a few options. One is to do with a less-best option.
Tile and grout are not waterproof. Moisture goes through grout.
tub flange: inside or outside your Greenboard?
more missing information: did you put your greenboard inside or only butting up to the tub flange? This is important.