Dielectric cavity mirrors for storage-ring free-electron lasers (FELs) in the VUV range have to overcome several challenges including resistance to synchrotron radiation and long-term stability in a contaminated vacuum. While oxide materials are not suitable below 190 nm, pure fluoride materials are not stable in this challenging environment. It has not been possible until now to generate lasing below 176 nm in a storage ring FEL. Duke University/TUNL and Laser Zentrum Hannover have successfully developed new strategies and produced mirrors with a high reflectivity of 95 \%, which enabled FEL lasing between 168,6 and 176,7 nm.