Holiday Season in Bulgaria, Vienna or Greece?

None of your three options will be warm at Christmas time! Depending on where you go in Greece, this is likely to be the warmest on your list, but it will be wet. You could get snow in any of the three places.

Bulgaria off the beaten track will be very, very basic. You can't do Vienna off the beaten track. Greece in winter, off the beaten track is likely to be closed up for the season.

If you want warmth, head for the Canary Islands - not Tenerife or Gran Canaria if you want off the beaten track. Somewhere such as Fuerteventura may suit, but you may have left it a bit late for the best choice of flights and accommodation.

Nowhere near as warm, but the Algarve in Portugal has a large supply of apartments, so it maybe easier to find somewhere here. Taking an apartment would be easier, as many places in Europe don't cater for vegans - particularly Greece and Eastern Europe.

New Year's Day is a holiday in many parts of Europe and therefore public transport maybe limited/non existent.

As noted above, none of the places you list above will be warm. If you really want warm, go to Morocco. Southern Spain might also work, but will still be cold. I find that in the winter months in Europe you find the most to do in ski towns or in large cities. Traveling to a mid-size city or village or rural area where most of the attractions are outdoors will leave you shivering and bored as many things will be shuttered. In a city there are often many museums and cafes where you can warm up. You, however, have the added disadvantage that your travel dates are essentially shut-down dates for just about everywhere as things are very quiet between Christmas and New Year's. In Vienna, for example, Christmas Markets start closing down on Dec. 24. That said, Vienna has probably more to do on your dates than any other place on your list.

In response to the post above regarding the Algarve, I would share that we spent an off season holiday in the Algarve in a rented apartment. It was horrible. Nothing was open, the apartment, which had sat empty for months, was moldy and dirty. There was absolutely nothing to do for entertainment either and the only restaurants we found open served bad German and bad British food.

Southern Spain would be a rather warm option, but I'm very doubtful that you can find a place off the beaten track.

Sicily might be a possibility, though I'm on the record as discouraging short trips there since there is so much to see.

The second poster must have been unlucky. I have been to the Algarve in winter and the apartment was good and the resort was "open". Some smaller resorts do close for the winter, so stick to the larger places.

Others have given you good reasons why Bulgaria and Greece don't suit your needs.

Vienna is a fine option but many things will close over the Christmas Holidays. In German-speaking countries, Dec 24th - Jan 1st is a time where almost no one is working, but rather visiting family or on a ski holiday. So some restaurants close that entire time, etc. But Vienna is a big enough city that you should be able to find plenty to do except for 24-26 and the 1st.

Given that you're not going to get warm except at a resort in north Africa or the Canaries, why not consider Alsace, France? The Christmas market in Colmar stays open until Jan 1st, and there are lovely little villages nearby with other Xmas markets to visit.