Is it obligatory to fulfill a vow by slaughtering a sheep every year, as was vowed, or is slaughtering a calf sufficient for the past years?

You must fulfill the vow. If your delay was due to an excuse, there is no sin upon you, but you must slaughter what you missed. If it was without an excuse, you must repent for your negligence. Do not substitute the sheep with other animals unless they are unavailable.