Infection of French-American hybrid grapevines by GLRaV-3 is latent, and information about the impact of the virus on the performance of these cultivars is scarce. This study compares several performance indices between Vidal blanc and St. Vincent grapevines that were naturally infected and those free of GLRaV-3. Virus infection did not affect grapevine vigor, but was linked to an approximately 5% reduction (p < 0.001) in average berry weight in both cultivars. Titratable acidity was 5 and 9% higher (p < 0.001) in the fruit from virus-positive St. Vincent and Vidal blanc grapevines, respectively, three to seven days prior to harvest. Also included in the study were Vidal blanc plants naturally infected by GLRaV-3 and grapevine fleck virus (GFkV). Plants with both viruses had a 7% reduction in average berry weight and titratable acidity levels were 14% higher compared to healthy plants (p < 0.001). Graft-indexing on Vitis vinifera L. cv. Cabernet Franc and Vitis rupestris Scheele cv. St. George indicators produced typical leafroll and fleck symptoms, respectively. These results suggest that latent infections in these French-American hybrid cultivars cannot be attributed to the low virulence of the viral strains, but probably to host tolerance.