Power Transformer Current Density

if the primary were in Star config, than the vendor would be correct...

No, if the primary were in Star (Wye), then the line current = the phase current, which would be the 87.47A figure. With a delta winding, the current from each line connection splits between two windings each way, per the 1/√3 factor, and you get the 50.50A figure. I believe the transformer vendor is correct.

May I ask why you are specifying the current density of the transformer's windings? That is usually a design element that the transformer manufacturer determines to give you the rating of transformer (MVA, %Z, physical size constraints, thermal ratings, etc.) that you request.

You are right and vendor info also correct

Any time a transformer is changing voltage between three phase power, there is no sq rt 3 calculation between primary and secondary. It is always 3 phase and your initial calculation was correct for the amperage.

If the system was being used to produce 3 phase from a single phase primary feed- not a transformer but an MG (motor generator) set. THEN you would look at the sq rt of 3 on the output (secondary) side and obviously there would be no correction on the single phase "primary".

Your calculation gives the line current. The primary being delta connected, the current through the winding is 1/sq root 3.The vendor's calculation is correct.

Amendment: My earlier answer was incorrect, as several others have noted. Thanks for catching.