Vitesse VSC8221 Ethernet PHY for Media Converter Application
The IC you mentioned (Vitesse VSC8221) is a typical 10/100/1000BASE-T/TX/T PHY with a 1000BASE-X/SGMII interface to the MAC. It is intended to design SFP modules, not to design media converters.

As i see you try to design a converter for 10/100/1000 Mbps copper to/from 1000 Mbps fiber optic. If it is so, you should use a switch instead of simple PHYs, because you need a speed matching mechanism (packet buffering memory) as the basis for "store-and-forward" concept.

The PHY you selected has no packet buffering memory and works like a "pass-thru" block: speeds on both its sides must be equal. It also does not manage CDMA/CD to deal with half duplex connections itself. It's only PHY and it needs a MAC over it.

Search for appropriate 2-port switch IC (often called "converter") with integrated PHYs (one is 10/100/1000 copper and one is 1000BASE-X serdes for fiber optic) for your task. Good luck.