When might van der waals forces be strong enough to two hold molecules together

When two molecules are close together.

Explanation:

Vander Waals forces of attraction and repulsion are forces of distance between molecules. The farther apart the molecules are the weaker the forces. Molecules, even non-polar molecules can have differential and instantaneous partial changes (that came and go in picoseconds called London dispersion forces). London dispersion force is even more significant in larger molecules. Molecules, polar and non-polar will therefore develop electrostatic forces of attraction or repulsion with neighboring molecules. These forces are responsible for surface tension in liquids and crystals in solids.