What does your Realtor say? I will be interested in knowing that. Hopefully he has already giving you a COMP (Comparable Market Anaylsis) on the property.
So, the originaly listing price is $390K, they dropped the price to $360K (how long ago is that?) and you want to offer $334K?
Is it a low ball offer - depedns on the comp, and how long has the house been sitting since the last price drop, how many houses are on the market that's cmparable, etc.
I agree with Cynthia, if you really want the house, make an offer on paper and present that to the buyers. Make the other terms sweet - all cash offer, high % of downpayment; short contingent period, not contingenty on the sale of your current house etc; then you just might have a good shot at it.
No, the house being vacant does not bring the value of the home down, unless it's not being maintained, then you have to be very careful when you do the inspections (which you should anyway); The market value really depends on the good old supply and demand.
As far as what Michale is saying, that this might be a Short Sale. I think we jump on that too quickly due to the current lending situation. However, this might not be a short sale. The original owner might have put a huge downpayment down or they might have paid off a lot of mortgage in the past 4 years, who knows. Unless your Realtor asks the listing agent, I won't worry too much about whether this is going to be a short sale or not.
So, ask your Realtor to give you a comp, make sure you are purchasing it at the price you want. Make sure your realtor is proficient and knowlegable to advise you with a good offering package and knows how to smoothly negotiate the deal for you.