Liturgical Handbook of the Russian Orthodox Church Abroad
Except when a blessing may be explicitly given for a singular service where the Bishop feels this might be a practical necessity based on unusual circumstances, 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 their cassocks, ryassas and any appropriate head coverings — taking special note that such head coverings (purple skufia, kamilavka, klobuk) are to be worn during the entrance prayers, and not to be set aside out of improper informality. (See the separate question on whether to wear a stole during the entrance prayers.)
This same rule means that when greeting the Bishop during an hierarchical service, the clergy similarly do not ‘pre-vest’ partially and greet the Hierarch with a ryassa covering vestments, but greet him in cassock, ryassa, and any appropriate headgear alone.