You are right. Hot water will flash into a 2-phase mixture of low pressure steam and water. However, the hot water must exist originally at a pressure higher than the one you are expanding it down to.
However, in the rest of your post, you are not expressing your application correctly and it is difficult to understand what you are proposing. For example, you state, “we pull a vacuum on our 140 F condensate, steam at ~ 3psig will flash into the vessel”. This cannot be correct as written. Water saturated at 140 oF has a vapor pressure of 2.893. So, if you have a vessel with only saturated water at 140 oF in it, the pressure in the vessel has to be 2.893 psia – which is a partial vacuum
. Also, where does the steam at 3 psig come from?
Then you make another thermodynamic mis-representation by saying “The vacuum pump will compress this 3psig steam to ambient pressure, 14.7 psia”. If the steam is already at 3 psig (which is equivalent to 17.7 psia at sea level), then you can’t compress it to a pressure that is lower. This doesn’t make sense and, I believe, is just bad communicating.
Are you simply trying to recompress low pressure steam? This is commonly done industrially – especially in multiple evaporation processes. Perhaps you can try to explain once more, in detail, just exactly what it is that you are proposing.
To obtain the thermodynamic values of Steam (& water) go to:http://webbook.nist.gov/chemistry/fluid/
where you can download all thermodynamic values free.