Viton is often used as elastomeric seat in butterfly valves. Viton is know to be restistant to 'difficult' chemical media.
However, in many chemical resistance lists the use of Viton is not recommended for hot water applications or steam service.
What is the specific danger of hot water ? Of steam to Viton ? Does it restart the curing process ? Does it attack the cross linked polyme
The manufacturer of the particular seal you are intending to purchase is also who to ask, and to find out if they have a clue: ask them "what is hydrolysis?", and also ask if they manufacture seals using Dupont trademark Viton rubber. 
Viton is a trademark of Dupont, and they claim to have formulations that can resist attack by hot water and steam.  I have no direct experience with these.  I do have experience with "generic" fluorocarbon seals, and they do deteriorate due to hydrolysis attack in hot water and/or steam.  Spoke at length with a gentleman from Greene/Tweed about the subject.  The hot water has a relatively high concentration of OH- radicals, which tend to attack crosslinks both between and along the polymer chain.  In general, a peroxide or platinum(?) cured Viton will be more hydrolysis resistant, and Dupont has some newer formulations for side chain polymers that also help.

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