A refrigerant system can be operated in either vapor injection mode or unloaded mode. A restriction to the vapor injection flow is incorporated into the refrigerant system. The restriction is placed on the vapor injection line upstream of a point where the injection line meets the unloader line. In this case, the flow is restricted for the vapor injection mode of operation to a desired level. However, when the refrigerant system operates in the unloaded mode, there is no restriction to the by-pass flow, which is beneficial to the system performance in this mode of operation. In one of the embodiments, the restriction area can be adjusted to achieve the best performance during the vapor injection mode of operation in relation to the operating conditions.