Temperature of a body can be positive, negative or zero. Is it a vector quantity?
No. Vectors are sometimes analysed by the sign. With the general convention that if B=<a,b,c> then -B=<-a,-b,-c>. I will assume you have been working with one dimensional vectors and that is what causes confusion.
If temperature was a vector then we would be able to give direction to temperature at a point. Suppose you are standing somewhere, and you ask someone, what's the temperature at your location, and they respond: “It's 30 degrees C south”. The south makes no sense because temperature cannot have direction.
It is, instead, a scalar. Scalars are simply magnitudes without direction. A scalar can be negative or positive or zero as well. But you never give it a direction. Examples include temperature, mass, etc. A good example of a vector in weather related concepts is “wind”. When people are predicting weather they will usually give the wind speed and wind direction. Other examples include forces, magnetic and electric fields, etc.