Thanks, i've overlooked that

    var mask = VectorMask.fromLong(SPECIES, 0b0111_1111);

    var i = 0;
    var upperBound = length - length % 15;
    for(; i < upperBound; i += 15) {
      v1.intoArray(result, i);
      v2.intoArray(result, i + 8, mask);
      ...