Have a look at this:
syms x [1,2]
syms y [1,2]
f(x,y) = x.^2 + y.^2
Notice that the resulting function is a function with four inputs, not a function with two inputs.
It is not possible in MATLAB to define a symbolic function that accepts vectors in positions and treats them as vectors in the calculations.
You can define
syms X
g(X) = X.^2
g(x)
That is, you can define a function that operates on scalar variables but when called with vectors does element-wise calculations (under restrictions -- there are definitely times this does not work out.) You can use vector-valued names in a function -- but if you use the vector-valued variables as the dummy variables standing in for what is being passed in, then the function will treat each one as a separate parameter.
You can use the relatively new symmatrix to write formulas in which the names are understood to stand in for fixed-sized vectors or arrays -- however, you cannot create a function from them. You can symmatrix2sym to convert to something that can appear on the right side of a symbolic function definition... but as soon as you convert the left side as well, you run into the problem that the names will each expand to a separate parameter.
