This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

First we want to define 3rd Party Vendor.  This question relates to creating solicitations for hotel/conference services.  A 3rd Party Vendor as used for this question refers to an offeror/bidder that is not the hotel/hotel chain itself but an independent contractor that after award contracts with a hotel to provide the required services. 

The language of a solicitation can require that the contractor have a physical presence at the function.  This is not an unreasonable expectation and not unduly restrictive.  We would suggest the language speak to the physical presence rather than a response time.  Any offeror can agree to a 30 minute response time (or whatever timeframe) and you will only be able to react after the fact (after award/missed performance).