Vantage uses the ESRI ArcGIS API for Adobe Flex to retrieve layers from ArcGIS Server. The API Supports the inclusion of tiled map services. In Map Studio (the Map authoring environment) you must set the tiled map setting to cached so that the map 'locks' to the levels of detail in the tiled map service. Otherwise, even when using a tiled service, the map user will be free to zoom to any level of detail. Zooming to any level of detail may be a required feature for a particular use-case. However, with tiled services, at levels of detail other than the pregenerated tiles, you will get a grainy version of the tiled service.

The latest version of the Vantage viewer supports 'grabbing the next set of tiles' This feature was included in the latest version of the ArcGIS API for Flex.

Let me know if this is the explanation you were looking for :-)