Is the Vivid app free?

No. Vivid offers a free trial, but the full app is paid, and it costs more than MacBrightness. The trial lets you preview the effect, while unlocking full brightness system-wide is a one-time license priced higher than MacBrightness's $5. Both are pay-once with no subscription and no account, so neither hits you with monthly charges; the gap is just the price tag. If you want the cheapest thing that opens the brighter range Apple holds in reserve, MacBrightness is $5 one-time. Its free Splitscreen Mode lets you confirm it works on your MacBook Pro 14"/16" or Pro Display XDR before paying a cent.