Actually, dictionary.tables is a virtual table created at runtime (by proc sql reading the files!) that is available only in proc sql. sashelp.vtable is a SQL view of this table.
Using dictionary.tables instead of sashelp.vtable can improve performance when you have a lot of libraries assigned with lots of datasets/views in them, as using it directly enables SQL to do a better optimization of the where condition. At least in my experience.
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