Traditionally you plant them outside the greenhouse but feed the main stem inside so that the grapes all have the heat and protection of the glass. If you haven't got soil in the greenhouse i would either dig a big hole - or use a huge pot - the roots will need a lot of room. you could get some topsoil in or use a mixture of loam and general compost. I would say the roots would want about 3 feet cubed at least. Give it some potash when it starts into growth, mulch each year with compost in spring or autumn and give it a flower feed (tomato feed etc) during the fruiting season. If the roots are restricted it will limit how much the plant grows.