what time of the year to plant my vitis vinifera Brant ?

I hope you got Vitis vinifera purpurea. Is Brant a named purple name? Not sure of my answer, but you might be as good to leave it in a good gallon pan and just store it in the garage or cold greenhouse.

Plant in the spring when the soil has unfrozen. I would have suggested planting in out if it had been Oct. or even Nov. I think it is too late now. The cold wet soil we have now simply is not conducive to root growth.

When you plant make sure it is in well drained soil.

My one gallon cans are being mulched with leaves as we speak. We are expecting VERY cold temperatures for over a week. The weather service for the Pacific Northwest has issued a severe warning.

I am in the States, but the commentary applies.

As it is such a small plant at the moment, I would be inclined to let it settle down in your coldframe or a sheltered corner for the winter, then plant it out in the spring. Give it a feed of blood, fish and bone when you do. If the weather is very cold, I would bubble wrap the pot to stop the roots from freezing.