I'm trying to rotate some vertices/PVectors using matrices.
As far as I understand, all I need to do is create a matrix, store the rotation, then multiply it by the PVectors.

If I create a PMatrix3D and use rotateX(HALF_PI), then multiply a PVector, I would expect to get the PVector rotated by 90 degrees. I did a simple test drawing a quad and a clone of that quad rotated using a PMatrix3D. The clone quad gets distorted and I don't understand why. Also, I've tried using ArbitraryAxisRotation matrices, one with rotations applied in x,y,z order and one with rotations applied in z,y,x order.
I've placed the 3 matrix rotation variants within //* //*/ blocks to make it easier to comment/uncomment a block by removing/adding a /.

My question is: how do I rotate a vector using a matrix in Processing ?

In my sketch, the red quad is the original quad and the green one is the one that should be rotated by 90 degrees. Here's my code so far:

1. PVector[] clone,face = {new PVector(50.0, 0.0, 49.999994039535522),new PVector(-49.999982118606567, 0.0, 50.000017881393433),new PVector(-50.000011920928955, -7.4505805969238281e-07, -49.999988079071045),new PVector(50.0, -7.4505805969238281e-07, -49.999994039535522)};
   color faceC = color(255,0,0),cloneC = color(0,255,0);
   
   void setup(){
     size(400,400,P3D);
     smooth();strokeWeight(1.1610855);noFill();
     clone = rotateVerts(face,new PVector(90,0,0));
   }
   void draw(){
     background(255);
     translate(width*.5,height*.5);
     rotateX(map(mouseY,height*.5,-height*.5,0,TWO_PI));
     rotateY(map(mouseX,0,width,0,TWO_PI));
       drawQuad(face,faceC);
       drawQuad(clone,cloneC);
     stroke(128,0,0);line(0,0,0,100,0,0);stroke(0,128,0);line(0,0,0,0,-100,0);stroke(0,0,128);line(0,0,0,0,0,100);
   }
   void drawQuad(PVector[] verts,color c){
     stroke(c);
     beginShape(QUADS);
       for(int i = 0 ; i < 4; i++) vertex(verts[i].x,verts[i].y,verts[i].z);
     endShape();
   }
   PVector[] rotateVerts(PVector[] verts,PVector rot){
     int vl = verts.length;
     PVector[] clone = cloneVerts(verts);
     PMatrix3D rMat = getRotationMatrix(rot);
     for(int i = 0; i<vl;i++) rMat.mult(clone[i],clone[i]);
     return clone;
   }
   PVector[] cloneVerts(PVector[] verts){
     int vl = verts.length;
     PVector[] clone = new PVector[vl];
     for(int i = 0; i<vl;i++) clone[i] = PVector.add(verts[i],new PVector());
     return clone;
   }
   PMatrix3D getRotationMatrix(PVector rot){
     PVector r = new PVector(radians(rot.x),radians(rot.y),radians(rot.z));
     //*
       PMatrix3D result = new PMatrix3D();
       result.rotateX(r.x);result.rotateY(r.y);result.rotateZ(r.z);
     //*/
     //using http://inside.mines.edu/~gmurray/ArbitraryAxisRotation/ArbitraryAxisRotation.html
     float a = r.x, b = r.y, c = r.z;
     //x,then y, then z
     /*
       PMatrix3D result = new PMatrix3D(cos(b)*cos(c),cos(c)*sin(a)*sin(b)-cos(a)*sin(c),cos(a)*cos(c)*sin(b)+sin(a)*sin(c),0,
                                        cos(b)*sin(c),cos(a)*cos(c)+sin(a)*sin(b)*sin(c),-cos(c)*sin(a)+cos(a)*sin(b)*sin(c),0,
                                        -sin(b)      ,              cos(b)*sin(a)       ,               cos(a)*cos(b)       ,0,
                                             0       ,                    0             ,                    0              ,1);
     //*/
     /*
       PMatrix3D result = new PMatrix3D(cos(a)*cos(b),-cos(c)*sin(a)+cos(a)*sin(b)*sin(c),cos(a)*cos(c)*sin(b)+sin(a)*sin(c),0,
                                        cos(b)*sin(a),cos(a)*cos(c)+sin(a)*sin(b)*sin(c),cos(c)*sin(a)*sin(b)-cos(a)*sin(c),0,
                                           -sin(b)   ,              cos(b)*sin(c)       ,           cos(b)*cos(c)          ,0,
                                              0      ,                    0             ,                 0                ,1);
     //*/
     return result;
   }