Diabetic Dermopathy: Given the patient's history of diabetes and the distribution of the pruritic rashes over bilateral forearms, elbows, back, and knees, diabetic dermopathy is a strong consideration. It is a common skin condition associated with diabetes, characterized by light brown or reddish-brown patches.

Atopic Dermatitis (Eczema): The pruritic nature of the rashes could suggest atopic dermatitis, especially if the patient has a history of allergies or asthma. However, the specific distribution and the patient's age might make this less likely than diabetic dermopathy.
Contact Dermatitis: Although there's no known history of recent drug reactions or allergies besides Vildagliptin, contact dermatitis remains a possibility, especially if the patient has been exposed to new soaps, detergents, or environmental allergens.
Psoriasis: Psoriasis can present with pruritic plaques on the elbows, knees, and back, and it has a significant prevalence in the general population. The patient's age and the distribution of the rashes make this a plausible diagnosis.

Bullous Pemphigoid: An autoimmune blistering disease that can present with pruritic rashes and is more common in older adults. It's crucial to consider this diagnosis due to its potential for significant morbidity if not treated promptly.
Cutaneous T-Cell Lymphoma (Mycosis Fungoides): Although rare, this condition can present with pruritic patches or plaques and is important to diagnose early due to its potential progression and impact on quality of life.
Drug Reaction (including Vildagliptin): Despite the patient's known allergy to Vildagliptin, it's essential to consider whether the current medication regimen could be contributing to the skin symptoms, either through a direct reaction or an interaction with other medications.

Necrobiosis Lipoidica: A rare skin condition associated with diabetes, characterized by necrotic, granulomatous lesions, often on the shins. While less likely given the description, it's a consideration in diabetic patients with skin lesions.
Granuloma Annulare: A benign condition that can appear as ring-shaped lesions and is sometimes associated with diabetes. It's less common and might not fully explain the pruritic nature of the rashes but remains a rare possibility.