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Let's do this with accurate equations. We know how to calculate the saturated water vapour pressure for temperature $T$: $$e_s(T)=e_{s0}e^{\frac{h_i}{R_w}(\frac{1}{T_{s0}}-\frac{1}{T})}=610\text{ Pa} \times e^{5423(\frac{1}{273\text{ K}}-\frac{1}{T})}$$ $e_s$ is saturated water vapour pressure as function of $T$ in Pascals $T$ is the temperature in Kelvins ...
The answer is easy. Replace $T$ with $T_d$, where $T_d$ is the dewpoint temperature. I will say I am not all that familiar with those equations, but that is besides the point.