If the bike is in as-new condition and the oil type is as specified by Vespa, i'd recommend you follow their advice .. otherwise it depends on the condition of the engine and oil type.
Personally i adjust oil so that i get little, if any, smoke and no oil dripping from the tail pipe. On disassembly i see a nice coating of oil on all parts. On a modified, high performance bike it might be wise to use more oil and have less concern for aesthetics..
a little bit too much oil is immesurably preferable to just slightly less than the engine needed..