I have used Vyatta and Cisco extensively, and I have probably completed one of the larger Vyatta deployments by a UK company (company WAN spanning 6 countries).
A few responses:
1) Vyatta vs Cisco, I am sure there is a point where the high end Ciscos out perform PC opensource based solutions, however at the commodity level this simply isn't the case. Bang for buck when you look at what you get with Vyatta feature and performance wise the equivalent Cisco is way way more expensive.
2) Vyatta vs Other Opensource, the argument is absolutely right, Vyatta does nothing that you can't get anywhere else on the opensource community, we actually use a Debian/iptables/imq solution elsewhere in our ISP business very successfully, and I would recommend if you are doing anything specialist you stick with the bespoke route. However Vyatta is extremely quick to deploy, give me a bare metal server, a Vyatta CD, and I will have an installed working Vyatta router with base config in 10 minutes, you simply can't do this with a custom build.
3) Cisco vs Opensource, long raging argument, we run our albeit small ISP all on Linux opensource routing equipment because its cheaper. Where Cisco wins though is support and the high end gear.
4) Support. Where Vyatta really wins is the support, if you decided go with Vyatta pay the subscription fee and get the support, its really superb.