Geothermal heat
Why do they exist at all? What in the hell keeps them warm if not geothermal venting?
It does not have do be "venting". Everywhere on Earth is a continuous flow of heat from interior to exterior. Normally it's completely swamped by the sunlight and day-night temperature changes, but several kilometers of ice is an awfully good insulator. Nothing impossible about Earth's internal heat diffusing through 40 km of solid rock and up into Lake Vostok at the same rate as it then dissipates through the ice.