Pentecost prostrations are not supposed to be done until the Vespers that evening, which marks the beginning of Monday according to the liturgical day. But since the presbyter knows that it is highly unlikely that anyone will come if he holds a service that evening, they usually do it immediately following Liturgy so that everyone does not miss out. So it is against the rules, but no one seems to care that much.
There are many churches that do not do the service immediately following Liturgy. But I'd say that the practice of doing so isn't more "against the rules" than, say, doing a Vesperal Liturgy (of Basil or Gregory) in the morning (as is done Holy Monday through Holy Thursday and Holy Saturday).
If the history of other "special" Sunday vespers services is a guide in my parish, we'll adjourn for coffee hour and breakfast after liturgy (around 11:00am) and around 12:30 those of us who haven't left will head back into church for (an early) vespers.
Sounds like a pretty good timeline; and gives an opportunity for people to get a bit of a rest before extending their Sunday Church experience (TM)
"O Cross of Christ, all-holy, thrice-blessed, and life-giving, instrument of the mystical rites of Zion, the holy Altar for the service of our Great Archpriest, the blessing - the weapon - the strength of priests, our pride, our consolation, the light in our hearts, our mind, and our steps"
Met. Meletios of Nikopolis & Preveza, from his ordination.