[Facts] Let`s simplify this
é (accent aigue) is not exactly pronounced like AY, a little narrower, but to help Anglophones understand we`ll just say it is.
è (accent grave) is pronounced like EH, as in bet. So is ê (accent circonflexe).
A straight e is pronounced like er, ur or ir minus the r.
When put in fornnt of certain consonants, the e and the consonant combine to form that same é/ay sound.
Vespasienne should either be Veh-paz-YEHN or Vay-paz-YEHN.