All stock and aftermarket FRPs I've ever seen for a Fox Mustang use a vacuum line to adjust fuel pressure based on engine loading.  That the fuel pressure is variable is assumed in the fueling tables programmed into the ECU.  Without a vacuum line connected to the FPR you will have running issues because fueling is only adjusted by the ECU from what is programmed in when the ECU is reading off the O2 sensors, which is only a tiny portion of the time with the EECIV.  All start up, warm up, etc, is done using only programming with zero input from the O2s. 
 

I would be sure to ask them specifically how the regulator compensates for the lower pressure required at idle, because without vacuum assistance the pressure will be too high and the engine will run rich at idle.  The only way to get around this would be with tuning.  Essentially the purpose of the vacuum line is to provide assist for the spring that opens the return line during low demand conditions.  The vacuum line on the stock regulator has nothing to do with boost, and this is not how a stock Ford EECIV system handles fuel enrichment under boost (it takes a reading off the MAP sensor and adjusts injector PW accordingly based on a table).  Aftermarket blower and turbo systems for years used a rising rate fuel pressure regulator or RRFPR that essentially did what these instructions are telling you to do.  Such systems have largely fallen out of use, as they are slower to react than electronic based systems and on a ragged edge tune up can allow the engine to go lean and nothing good happens when that happens under boost. 

According to the tech support guy "if the original fuel pressure regulator had a vacuum line hooked up then this should have a vacuum line hooked up too".  I asked why this is in direct contrast to the instructions provided and he said those are 'referring to something else' even though this exact part number is listed for those instructions. 

Doesn't exactly instill a lot of confidence that they know what they are doing.

TremendousWand wrote:

According to the tech support guy "if the original fuel pressure regulator had a vacuum line hooked up then this should have a vacuum line hooked up too".  I asked why this is in direct contrast to the instructions provided and he said those are 'referring to something else' even though this exact part number is listed for those instructions. 

Doesn't exactly instill a lot of confidence that they know what they are doing.

Uh, its a regulator that only fits the fuel rails on a Ford.  All Fords had a vacuum line connected.  Translation: we wrote one set of instructions for all the regulators we sell, but couldn't be bothered to check that the changes we made to the universal instructions were actually accurate for each application specific regulator we sell.  
 

MS wrote:

My MSD ATOMIC EFI system I had on my 427 came with a regulator that had the vacuum port. I also followed the instructions and left the port open since my engine was normally aspirated.

I got rid of the EFI because it had a problem where the fuel pressure would go to six psi during high rpm hard acceleration, leaving me on the side of the road not running for five minutes, until something reset and it would restart.

After reading previous replies here about the regulator port, I am wondering if that had anything to do with my flame-out condition.

My 750 Holley has worked perfectly since the efi was removed, but I lost half the cost of all that efi stuff.

Anyone know?

I can't say for sure, but the vacuum line on a Ford EFI system does nothing to increase pressure under WOT, where there would be no vacuum.  Its only to provide assist to reduce pressure under conditions of high vacuum.  There are different types of regulators though that operate differently.  The reset after not running for 5 minutes would lead me to believe that the issue was electronic in nature, but hard to say for sure.