Boots of Haste

The Boots of Haste, not to be confused with the D&D 5e Boots of Speed, are an equipment that can be clicked together to cast the Haste spell on the wearer. They require attunement.

Properties
The boots in Critical Role seem to be a house-ruled bastardization of the Boots of Speed from D&D 5e with the ones from Pathfinder. Vax's boots follow the D&D rules for activation, but have the Pathfinder effect of casting Haste. Based on that, they would have the following properties:

''Requires Attunement While you wear these boots, you can use a Bonus Action and click the boots' heels together. If you do, you benefit from the effects of the Haste spell for the duration of this effect. If you click your heels together again, you end the effect. When the boots' property has been used for a total of 10 minutes, the magic ceases to function until you finish a Long Rest.''

Note that, unlike the actual Haste spell, the boots' effect does not require concentration and lasts ten minutes as opposed to only one.

Vax's Boots of Haste
Vax'ildan owns a pair of these boots that have caused a long-standing competition between him and his sister, Vex'ahlia. The feud began when Vax loaned the boots to his sister and she took a liking to them. Ever since, she will steal them from him at every opportunity she gets. Vax typically reclaims them within a short time, though sometimes only because his sister takes pity on him and gives them back.

Trivia

 * CritRoleStats has examined which of the twins actually gets the bigger benefit from these boots in Who Wore It Better: Boots of Haste.
 * Matthew Mercer has stated that the boots are a broken item due to how they get around the action economy of D&D 5e. He has also made suggestions as to how they could be better balanced.