TRACON Controller reported an aircraft cleared for an approach descended below the Minimum Vectoring Altitude. The controller trainee's incorrect phraseology contributed to the altitude deviation.

Narrative

I was training a developmental. Aircraft X was vectored to join the ZZZ radial for the VOR approach and instructed to maintain 3000 ft. Their altitude at the time was 4000 ft. The phraseology was issued incorrectly as the trainee said 'Aircraft X 15 miles from ZZZ VOR turn right heading 290 maintain 3000 cleared VOR approach' leaving out until established on the approach. I discussed this issue briefly with the trainee as we were working a 3 plane sequence to intersecting arrival path runways and Aircraft X was number three. Instructions needed to be issued to other aircraft; but I should have made a point to have the phraseology corrected on frequency. During my scan I saw Aircraft X join the ZZZ radial about 14 miles from the VOR for the VOR approach at which point they could descend to 2500 ft. until 10 miles from the VOR where they can descend to 1800 ft. According to our function this is just inside of the depicted obstacle in a 2400 ft. MVA under the approach path. As Aircraft X began descending I noticed they were coming down quickly and told the trainee to pay attention to them. As Aircraft X crossed the obstacle they descended 2400 ft. to 2200 ft. pretty quickly at which point I told the trainee to issue a low altitude alert; which unfortunately was issued incorrectly leaving out the LOW ALTITUDE ALERT phraseology. I didn't correct the phraseology as Aircraft X was now inside of what the function showed to be 10 miles from the VOR and was legal to descend to 1800 ft. So; I had the trainee tell Aircraft X to descend via the approach. A brasher was issued for a possible pilot deviation. During the tape talk I had with my Front Line Manager (FLM) yesterday; I was advised that the management believed this was a significant loss with terrain on me the Controller and was advised to file a report. I am not sure how accurate the function on STARS is compared to the DME readout on aircraft; so this may not have been an MVA bust or a pilot deviation. However; there is no marker on my radar map that tells me exactly where 10 DME is from the VOR on that approach. I recommend a map update to reflect that so that we can avoid issuing erroneous low altitude alerts. Also; the approach plate's REMAIN WITHIN 10NM has always been open for interpretation as to whether it is 10NM from the VOR or the airport. I recommend clarity of this distinction on the approach plate.