To add a vacuum gauge you will need:

A vac gauge with 1/4" NPT threadshttps://www.amazon.com/HFS-Utility-Vacuum-Pressure-Blk-Steel/dp/B00VQSP2XA/ref=sr_1_3?keywords=vacuum+gauge&qid=1581185089&sr=8-3

2x 1/4 hose barb to 1/4" NPT:https://www.amazon.com/gp/product/B002SAO7XQ/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1

1/4" NPT T fitting:https://www.amazon.com/gp/product/B000BQUTBS/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1

Teflon tape to seal the threads of NPT fitting:https://www.amazon.com/gp/product/B06XCWQ4D2/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1

Setting it up is straight forward. You just need to make a clean slice in the vacuum line a few inches before the pump. Make sure you leave enough on each end of the hose barbs.