I went to OCA vespers services and the priest took off his phelonion several times during vespers SAT NIGHT
I have never seen this before - he explained that it was a russian orthodox rubric
Can anyone tell me if Serbian Orthodox do the same?
Vesting and unvesting during Vespers?
I think what you are seeing is the common vesting procedure when serving a Vigil. It seems to be a common practice amongst some of the clergy in the OCA to serve Great Vespers as if it is part of the Vigil service, even when it is not being done as part of the Vigil service.
The priest starts off wearing the Epitrichilion and Phelonion - for the great censing of the church (properly done before the service starts)
After the "Blessed is our God..." the Phelonion would be taken off.
The Phelonion would then be put back on before the little entrance is made.
The Phelonion is then removed after the Aposticha.
I have seen the Phelonion put back on for the dismissal hymns, but that is unusual. In the Vigil the Phelonion would not be put back on until the Polyeleos, then after the Gospel reading the Phelonion would not be put back on until the Great Doxology.
When liturgy is celebrated with the Vigil (as is proper in serving a complete vigil) the complete vesting takes place during the singing of the canon. Even then the phelonion is not usually put on until before the Great Doxology.
When Great Vespers is served by itself there is no Great Censing prior to the service starting, so the priest should begin the service in only his Epitrichilion.