The advantage in making a shirt then splitting it is that the front and back halves will match with little effort required to turn into armor plate by removing vertical faces at the spit to create space between the two halves. A few minor adjustments, make the straps, UV map, texture and you are done.

You can use the shrink wrap modifier on the shirt model if you like before splitting.

To split the model into front and back halves:

In EDIT mode press 3 then the decimal on the NUMPAD to frame the model in a side view.

Select either the front or back half of the shirt, press P then SELECTED PARTS in the dialog.

You will have matching front and back objects to use as the basis for the armor.

If you want to make the armor have actual thickness let me know and I'll point you to the scripts that you will need to make that easier to do.