Transient errors caused by particle strikes have become a critical challenge for microprocessor design. Being the major consumer of on-chip real estate, cache memories are particularly susceptible to transient errors. However, not all cache soft errors can be propagated to the processor. For instance, soft errors can be corrected by write operations before they are read. In this paper, we define the cache vulnerability factor (CVF) to be the probability that a fault in the cache can be propagated to the processor or other memory hierarchy. We also propose an approach to compute the CVF based on the cache line access patterns. Building upon the CVF, we evaluate the reliability for different cache memories. Our results show that 83.5% of soft errors from a write-through data cache can be masked without affecting other components. We also propose two early write-back strategies to improve the reliability (i.e., by reducing the CVF) of write-back data caches without compromising the high performance.