The fact that you used it for business (farming) most of the time that you owned it, means that you can treat it as the sale of investment property (not personal property) and deduct the loss (a capital loss is limited to $3000 per year, the rest will carry forward to future years).
For your cost basis, you will have to use your best estimate to assign the property amount of your original lump sum purchase price to the vacant land. Since the properties were separate tracks, the original (11 years ago) tax appraisals should provide you the correct ratio.
For example: you bought the 2 properties together for $120,000. The tax appraisal for the house was $60,000 and the farm land $40,000. 40% of your purchase price can be allocated to the farmland. 40% x 120,000 = $48,000.
