He also said two mattresses were left around the property and a former tenant was telling people the house is now infested with bed bugs.  

Im in contract to buy a cat infested condo.

Whatever you decided to get, do NOT forget to buy zippered mattress encasements to help prevent bedbug infestations.

However, if it has never been tested, you don't know for sure and do not have to claim that as a seller.If the seller DID have knowledge of it, whether that is through their own experience with the issue or a contractor informing them, they can be held liable and the buyer even has means to sue them for damages.If you are a seller, and whether it is a bug infestation, gas leak, foundation issue, etc, do the right thing and claim all of these on your seller property disclosure.

Vacant as in blighted, abandoned, condemned, infested, etc.

And considering that PA is infested with slumlords and gullible students and internationals willing to pay for high market rates for low quality like in NYC (without the NYC opportunities or amenities), it's not surprising that Sec 8 is simply not as popular here. 

Neither I nor the pest control guy believe an infestation like this could have happened in such a short period of time.   

Trash and rat infested backyard.

Here in Georgia, termites are very prolific and most houses that have been around a while have some infestation history.

In fact, I specialize in taking over properties that are infested with drug dealers; kicking them out; and rehabbing the buildings.