That connection usually works. Bulgarian railways are mostly pretty well on time and this is an official connection. Border control takes place at the entrance to the platform, so directly after arrival go there and wait. I think they start around 15mins before with the border control. Usually you can count the amount of passengers using this service on one hand so it shouldn’t take too long. The few times I was in Vidin, the connection would have always worked. 

if you are afraid of missing it, then take the train the evening before to Vidin and sleep there. Vidin is quite nice as well. Just probably in winter you will miss the beautiful views until Mezdra (from Sofia). 

thx for quick reponse. My limited exp on Bulg rail has not been so good re punctuality. On 3 hr trip to Plovdiv they were 25mins late, so I was worried it would be the same or worse on a four hr ride. Perhaps the connecting train will wait. Anyway, based on your experience  I,ll take a chance.

Good luck! As said when I saw this train/used it, the connection would have worked, but no guarantee unfortunately.

if there is a delay, for some time the connecting train should wait I suppose.

You were totally correct, Bulgarian rail was punctual and we made the connection, ad indeed we did for the short transfer time at Arad, thx to Romanian rail！