The value of which property decreases with an increase in the strength of intermolecular forces?

$ \textbf{(A)}\hspace{.05in}\text{viscosity}\qquad\textbf{(B)}\hspace{.05in}\text{boiling point}\qquad\textbf{(C)}\hspace{.05in}\text{surface tension}\qquad\textbf{(D)}\hspace{.05in}\text{vapor pressure}\qquad $
1. **Analyzing Choice A (Viscosity):**
   - Viscosity is a measure of a fluid's resistance to flow. 
   - As the strength of intermolecular forces increases, the molecules in the fluid are more strongly attracted to each other, making it harder for them to move past one another.
   - Therefore, an increase in the strength of intermolecular forces leads to an increase in viscosity.
   - Hence, choice A is incorrect.

2. **Analyzing Choice B (Boiling Point):**
   - The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure.
   - Stronger intermolecular forces mean that more energy (higher temperature) is required to separate the molecules and convert the liquid into vapor.
   - Therefore, an increase in the strength of intermolecular forces leads to an increase in the boiling point.
   - Hence, choice B is incorrect.

3. **Analyzing Choice C (Surface Tension):**
   - Surface tension is the energy required to increase the surface area of a liquid due to intermolecular forces.
   - Stronger intermolecular forces result in a higher surface tension because the molecules at the surface are more strongly attracted to each other.
   - Therefore, an increase in the strength of intermolecular forces leads to an increase in surface tension.
   - Hence, choice C is incorrect.

4. **Analyzing Choice D (Vapor Pressure):**
   - Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase.
   - Stronger intermolecular forces mean that fewer molecules have enough energy to escape from the liquid phase into the vapor phase.
   - Therefore, an increase in the strength of intermolecular forces leads to a decrease in vapor pressure.
   - Hence, choice D is correct.

The final answer is $\boxed{\text{D}}$