Reginald,
If your need for a VNA is driven mostly by EMI filters and measurement, then I'd say you don't need a VNA.
A spectrum analyzer (in conjunction with some appropriate probing devices and methods) is most useful for tracking down EMI pathways, and measuring effectiveness of various cleanup techniques. You can't readily design or predict the in-service characteristics of say, a power line filter, with VNA measurements. This kind of filter is not an RF/microwave device with nice, single-ended 50 ohm I/O, and it goes in a non-RF environment.
Ed
