can i do vec3( matrixrotation * vec4(vertex, 1)); to get a vec3? matrixrotation is a 4x4 matrix. thanks 
Yes. For a rotation, you can also use mat3x3(matrixrotation) * vertex. But that’s only equivalent if the matrix has no translation component.
If you want to specifically ignore the translation component (e…g. if you’re transforming a direction rather than a position), you can use the mat3x3() approach or set the fourth component of the vector to zero rather than one.