Hello Suzy! The reason vaseline should not be used when a patient is on oxygen is that vaseline is a petroleum based lubricant which can be more flammable if near oxygen. Another reason, and more important, is that the vaseline on any of the oxygen equipment can actually cause the equipment (such as the valves, washers, etc.) to leak or breakdown.
There are some great guidelines on the web on the use of oxygen. The information has been developed by the Hospice of the Florida Suncoast, and are available at:
http://www.thehospice.com/oxygen.htm
There may be a connection with the vaseline, but several other things could cause sores on your father's face.
The mask, perhaps in combination with the moisture related to it, may have caused friction and skin breakdown. Areas of damaged skin are at risk of developing infections.
It may be possible that the vaseline is something your father is sensitive or allergic to. Sometimes rubber/plastic products (such as the mask) may contain latex, and a person may be sensitive to the latex in the product which can cause a rash to the area where the product with latex touches.
People may develop facial rashes as a side effect of medications or from a viral rash on the face called "shingles".
Hope this helps,
Simone Stenekes
Clinical Nurse Specialist
Canadian Virtual Hospice