Handbook of ROCOR Liturgical Practice:

Is it permitted to partially ‘pre-vest’ by donning the podrisnik (alb) and other vestments, covering them with the ryassa, prior to the Entrance Prayers?

Except when a blessing might be explicitly given for a singular service where the Bishop feels this might be necessary, as a standard rule it is categorically forbidden to ‘pre-vest’ in this way. The Entrance Prayers are to be said before any vestments are donned, with the clergy clothed only in ryassas (see the separate question on whether to wear a stole during the prayers).