Mate,
The vessel / tanks are governed by the pressure vessel code AS1210 which allows the design to full vacuum, ie -101.35 kPa (approx. 15 psi) or by the API 650 / 620 which allows only a small vacuum in the design conditions, up to -1 psi. I'm not sure where is the 5 psi coming from, so I am unable to comment on that. Hope this helps a bit.
Cheers,
gr2vessels