Ah, yes. The old Vaseline in the boot trick. I will say the notion of spreading Vaseline on the liner socks is an updated version of this hoary old tale, which in its earlier incarnation called for a hiker to apply it directly to the skin. I first heard of this in 1976, when some friends I were hiking on the coastline of Olympic National Park thought they would very cleverly eliminate blisters by slathering their feet with Vaseline. It didn’t work — their socks absorbed most of the stuff, and they blistered anyway. To make matters worse, just enough Vaseline stayed on their feet to make it impossible for tape or bandages to adhere. So they had to suffer through the rest of the hike with greasy, sore feet.
The idea, of course, is that the Vaseline reduces friction, friction causes blisters, so no friction equals no blisters. But I can’t see any advantage to putting it either on your feet or on your socks, except as perhaps a short-term emergency measure. For one thing, I don’t think it really works as a prophylactic against blisters, and it can cause complication such as what I mentioned above. It will ruin socks — you’d need to wash them 10 times to get the gunk out. And if Vaseline gets into your boots it will absolutely ruin them by softening the leather and possibly causing the cemented midsole to de-laminate.
In short, the best solution is make sure your boots fit, then wear proper liner socks. Forget the Vaseline, except for…oh, never mind.