I am thinking about travelling to Serbia, Albania and Croatia in december; will I have any issue as a Canadian assuming I am vaccinated?
I don't think anyone can predict what the situation in December will be.
I just came back from a trip to Europe 2 days ago. While it's still "doable" there is already a lot of talk about increasing restrictions again and there is a lot of concern about the recent uptick in Covid cases.
In Serbia, for example, new cases where down to less than a 100 at the beginning of the summer but now it's back up to over 4000. Given such a dramatic rise it's reasonable to expect some significant changes to travel rules, especially of the rise continues like that.
I'm going to disagree with Hilmar's answer. While its certainly true that any country can shut borders at any time due to COVID policies, the countries on your list have been some of the most open for tourism during most of 2020 and I seriously doubt they'll shutdown this year all of a sudden now that we have vaccines available. Plus there’s more and more understanding of the fact that travel bans don’t work, unless they’re done with hotel quarantines as in Australia.
Serbia allowed anyone to visit since May 2020. They were frequently used as a "travel bridge" to the US for this reason, as Europeans were banned from the US since March 2020. Albania reopened in June 2020 and stayed open - they did ban flights from the UK for a while but were otherwise very much accessible for all. Croatia reopened in July 2020 with the only requirement being a PCR test and proof of booking accommodations - it was frequently used as a "travel bridge" by Americans seeking to visit Europe last year, as it was the most convenient loophole for those wishing to enter the Schengen area.
Overall I'd say there's a 90% chance you'll be able to visit all three countries and a 99% chance you will be able to visit at least one country out of the three in December.