this was never a feature. you can't go past 100%. You can, however, normalize the gain on that clip, and then use the gain handle to bring it down to where you need it.
Despite Event's gain you can set the track volume in the audio's track headers up to + 12 dB and the track volume envelope up to + 6 dB (in all Vegas versions including VP16).
Don't know about the latest VP versions but the 'event' gain always maxed out at 0.0dB (no attenuation, all the way up..100%) As was stated there are a few other ways in increase volume, event normalize track volume, track/sub/master volume and envelopes, volume maximizer plup-ins.