Are vaishnavas allowed to donated blood, if no what is the prayachitam I can do after doing the blood donation.?

Answer: As in those Vedic Era, there were no such practices existed as that of in todays body part transplantations. There is no mention about blood donations of kind in any of the scripts; excepting the Asuras & Demons drinking the human blood, there is no mention about this specific issue. However, in the modern developed world, it is still valid to donate blood & after death donating organs such as kidney & eyes are surely helpful to other needy mankind. Though, the learned schollars and seers are of different view and oppose to it, it is wise to use ones own instinctions in such matters. Prayaschitham and pariharam can be found for this too! Performing Gayatri Japam and and doing it with holy fire (Gayatri Homam) alone should suffice for such deeds, as the donations are knowingly offered to other person for some life saving activities. Donating blood and body part is certainly not against the humanity. So one can ; ahead and donate blood & body parts, if one wishes. (Ekaliva offering his Thump as Guru Dakshina is not falling in this category!).