I do not have exact minimum requirements, the technical spcecs call for 1Gbs.
Vulnerability Assessment runs many simple select statements against the database and database manager.
This process could also be throttled by creating multiple small jobs versus trying to test all at once. Low bandwidth should be okay.
Data Protect/Auditing can be extremely busy or a trickle depending on the servers being monitored and the policy in place.
I would strongly recommend Case 2, the local collector. Normally a collector will export to the Aggregator once per day, but if this overwhelms the bandwidth, increase the frequency of the exports/imports. It may take some tweaking, but I believe you could obtain an acceptable level.
Thank You,

Jody Hair | Technical Architect – Data Security
Mobile: 972-510-5639
Twitter: @hairraiser | LinkedIn: /jdhair
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