I can't wait to fly a fleet Vesper myself... I just love how cryptic made 3 versions of each ship... a basic original design, a "rugged" older design, that's still not too old, but probably have been in service for a good 20-30 years and some brand new few year old models.
So yeah, can't wait to fly a T5 Vesper/Excalibur. There's T5 escorts, science vessels, T5 Cheyennes ()... give the Excalibur/Exeter/Vesper some love, please!
I would like to know if cryptic could dance around the official message and release a tier 5 of whichever connie did not get the mandate, as for op's subject there is no excuse for the lack of fleet exeters. When the fleet Exeter does come, it will be woefully underpowered just like all the other low tier fleet conversions sadly. Won't stop me from pissing of my teammates though.
The Somraw, K'tinga, D'Kyr, D7, Kumari, Xindi carrier, Xindi escort, and the T'Varo are all older than the Constitution Refit and yet they are tier 5. The Constellation is made up primarily of Connie refit parts and it is tier 5, there is no logical reason whatsoever for the no tier 5/6 connie rule.
Logically the Vesper and Excalibur are "new" starships, so I wouldn't see why not. But personally rather see a new T5 starship that takes omage to the Consitution, but much larger. (Though far from the JJPrize).