Powering the Vdd pin on the 4511 with 7.5V instead of 5V is the place to start. However, this might create a couple of problems which may/or not prevent it from working...
Raising the supply voltage on the 4511 while still operating the PIC on 5V means that you have reduced the margin on having the 4511 recognize a logic one (Vih) on its inputs when the output from the PIC only pulls to a Voh of 5V.
Hopefully, the voltage drop through two LEDs in series in each segement will prevent the PNP transistor from turning on while the anode of the LEDs is at 7.5V. The emitter of the transistor (Cathode of the LED segement) cannot be higher than one Vbe (~0.6V) above 5V, other wise the segment will be partially on while it is supposed to be off.
Try it and see if it works. Make the 7.5V adjustable. It may work at 7V, even if it doesn't work at 7.5V. You cannot raise the Vdd on the PIC above 5.5V.