Is the pilot required to keep visual lookout during IFR operations in VMC?
You didn't mention which country or jurisdiction you're asking about, but in the US 14 CFR 91.113(b) says:


  When weather conditions permit, regardless of whether an operation is
  conducted under instrument flight rules or visual flight rules,
  vigilance shall be maintained by each person operating an aircraft so
  as to see and avoid other aircraft


I assume other countries have similar regulations but even if they don't, it's still common sense and good piloting to keep a visual lookout whenever possible.