Even though I hardly relate this question to
TeX fora it is still a question of high importance in the field.
It doesn't even seem fit on sister site Math StackExchange
And some kinds of questions are considered off-topic:
Even though they refer to here...
In general there is no applicable convention. You are free to use what ever you see fit.
I have always used the bold type. Not only have I encountered this in the majority of my learning books, I also think that it seams fit to use that convention. It is short and concise, yes, in some cases it can be hard to see the difference if all variables are vectors, but you cant clarify everything for the reader.
The arrow I don't like. Most of all due to the complexity of writing a vector transposed, or maybe a vector to the second power and then transposed. The more you stack on top, the uglier it gets... In my opinion.
Complex numbers I just usually denote with
c which in typical school books are used as complex values. There is to my knowledge no standard on typesetting complex numbers. Just denote it by
Complex vectors I handle the same as real vectors.
As has been said, the main thing is that you need to be consistent! This is the most important thing.