The very short answer is that (as you mentioned) vo2 max is anywhere from difficult to physiologically impossible to increase, but lactate threshold (LT) is highly trainable. LT is often expressed as a % of vo2 max, with a higher value generally indicating that you are better trained, and able to utilize more of your potential power over a given period of time. Of course, the response to HIIT sessions designed to increase your LT varies. Some people will not improve much at all, while pro athletes will see incredible gains, but most of us are somewhere in the middle. It's also important to know that more HIIT != better; the quality of what you do, and recovering well is what matters most.