Vasicek
{% dr = (b - a r) dt + \sigma dW %}
Calibration
There are three parameters to the Vasicek model. Therefore, unless the parameters are pre-sepcified, it will require three zero coupon bond prices to fit the model.
However, while the model may fit the 3 selected prices, it will not fit the entire curve, only the selected bonds. This limits the ability of the modeler to calibrate the vasicek model to a given curve. For some applications, this lack of fit may be acceptable.