Vivaldi app icons get installed to /opt/vivaldi/product_logo_{SIZE}.png instead of /usr/share/icons/hicolor/{SIZE}/appname.png firefox-bin ebuild has a nice simple for loop in src_install for the same issue. Reproducible: Always
Please try with www-client/vivaldi-1.0.94.2_p2-r2.
Great, icons show up as expected now. Thanks!