Seems quite clear-cut to me.
All you have of the Vault Dwellers is a message on a terminal. You don't know if that message is hour, months, or decades old. There's no knowing if any of those people are still alive (un-feral ghoulified no less, unlike the rest of the in the vault).
However letting that reactor run irradiates water going to the farms, which means more people will suffer and starve, as it would seem that the Sharecropper Farms is the biggest food production operation in the entire Mojave. Also, hungry soldiers aren't good soldiers, which means NCR would have even more trouble against the Fiends and Legion (which may be a good thing if you plan on supporting the Legion).
On the other hand my current character stumbled upon the vault by chance and didn't know of the farms' situation, and thus saved the vault people (too bad she couldn't read the floating text on the screen

)
I really don't see a "Good vs. Evil" decision here, both choices can be both depending on your motivations.