I think you will get different answers from different people, even from different priests. The original intention for the vigil Mass, as you stated, was to help people like firefighters, police officers, etc., who cannot make it to Mass on Sundays for legitimate reasons, to be able to make it to Mass. Over the years, it has evolved into its current form. When I was going through RCIA, I was told that it’s fine to go to vigil Mass instead of on Sunday. To be honest, I’ve always felt weird going to Sunday Mass on a Saturday, but have done so on many occasions due to its convenience. But then again, Mass is not about our convenience, and for those of us who can make it to Mass on Sunday, I honestly don’t see a reason why we shouldn’t make an effort to do so.
Also, as a side note, there are no vigil Traditional Latin Masses offered on Saturdays, which are on Sundays only. I may get quite a bit of flak for saying this, but a novus ordo priest is more likely to tell you it’s okay; whereas a traditional priest will more likely tell you that you should make it to Mass on Sundays if you can.