We know very little about the
Valiant. We know she disappeared, presumed lost, about two centuries prior to the events of WNMHGB. We know she was designated as a Galactic Survey Cruiser. We know she was deliberately destroyed by her own crew.
We can assume she had some form of early FTL capability, but we don't know her speed and range capabilities although we can assume they were quite limited compared to that of the
Enterprise. We don't know whether she was a sleeper ship or not. We also don't know her crew complement.
Her designation as a Galactic Survey Cruiser suggests her mission profile was to range some distance from home port for an extended duration and possibly to visit multiple star systems before returning home. If so then that implies a vessel capable of operating independently from any outside support for extended periods of at least several weeks, but more likely several months and perhaps even a few years. We don't know if she was armed because we don't know when Earth first made contact with aliens and whether they were friendly or not.
We don't know if the
Vaiant was a one-off design or if she was of a class of similar vessels. And finally we don't know how long she was in service before she disappeared--did she disappear relatively soon after her launch or was she operational for some years before disappearing?
As an early exploratory vessel it's possible the
Valiant and her kind was something rather significant in her time. In the early days ships failing to return home were likely not rare, but the loss of a ship like the
Valiant could still have been quite noteworthy. Ships like the
Valiant were likely the vanguards in determining how dangerous interstellar space was and laid much of the groundwork for colonization ships to follow.
A lot of potential conjecture here.