what is the difference between a vigil ( on Saturday night for example) and mass ( on Sunday)? Is it acceptable to go to one instead of the other??
The vigil is the Mass of anticipation for the following day. Going to a vigil Mass on the eve of a Sunday or a Holy Day of Obligation satisfies the obligation.
There is no difference between a Saturday evening vigil and a Sunday Mass. The same readings are used. It is the same Mass. There are some feasts (Christmas and Easter are the only ones I can think of off hand) where the readings for the vigil will be different from the readings on the feast itself (and the readings on the feast itself might differ depending on what time of day you go). However, even in such cases, the attending the vigil fulfills the obligation.
Just to add to what atsheeran said, the Vigil Mass actually has its roots in Judaism. The Jewish method of reckoning days was one sundown to the next. Thus the Sabbath began at sundown the previous day. In the Catholic Church, the Vigil has been extended to include Masses said in the afternoon of the previous day, all of which meet our Sunday/Holy Day obligations.