I own a water-cool Volkswagen Vanagon GL 1990. Is it ok to mix different types of antifreeze?

Actually, it's no okay. Due to the combined magnesium case and aluminum heads, the galvanic reaction between the two metals creates some serious corrosive problems in the Vanagon engine. You really want to get pink stuff...it has the best inhibitors. Next to that is the orange stuff from Prestone. Also, you need to brush up on the fill and bleed procedure for the coolant system. Check the Bently manual and/or Vanagon.com Additionally, Water Wetter is a great additive for removing tiny air bubbles from the coolant system. And lastly, you want to completely flush the coolant system every two years. I recommend swapping out the thermo-switch then too. I learned these lessons the hard way.