What are the parameters for the control valve? Depending what the inlet and outlet pressures are the the vapor pressure may not be critical and in many cases, you should be able to come up with a reasonable guesstimate by looking at the process.
Fisher's control book has a good section on cavitation and how to use it to correct for it with the standard liquid sizing formula. In lots of cases, the cavitation doesn't really impact the valve sizing but is an indication you may want to look at hardened trim to prevent internal damage.
For example, if you were sizing a minimum flow control valve for a crude charge pump, you might be sizing the valve with 300 psig or more on the inlet and a few psig on the outlet if you are going back to a storage tank. In that case, I'd spend more time coming up with a reasonable vapor pressure than if I was sizing a control valve for the crude train where I have 300 psig inlet and 250 psig outlet. In the latter case, the vapor pressure isn't a significant factor.
I'd be cautious using Hysys. The components that drive the vapor pressure are the light components that are the smallest percentages and the easiest to lose during sample collection and analysis. I've seen liquid samples that would be at their bubble point taken from vessels that when you put the composition back into Hysys as a bubble point liquid with either a pressure or temperature, Hysys doesn't come close to matching the temperature or pressure. Similar comments go for vapor samples but where you are trying to calculate a dewpoint.