Regarding the VA, it's difficult to speak in generalities, as there is tremendous variance in the quality of these jobs. Some VAs are associated with high-quality university programs, so you are - for all intents and purposes - a faculty member. Then there are VAs that are so horribly run and in undesirable locations that it requires one to live in a state of quasi-dissociative fugue just to make it through the day. That can be difficult to do, especially considering it could take you up to 30 years to get your pension.
The production expectations at a VA are undoubtedly lower, and it's virtually impossible to fire someone after making it through the 1-year probationary period, so job security is about as good as it gets. That said, there's a stigma attached to working at a VA, so you may find it more difficult to switch to private practice if you ever decide to leave.
The only other major federal player in radiology is the military, and most of those positions are for independent contractors as opposed to employed positions. The production requirements and bureaucracy headaches are similar to the VA. The pay tends to be better, but the trade-off is that job security is virtually nonexistent. That's not to say that government contract radiologists are routinely let go, just that they could be.
With respect to the pension, I think that's an undervalued part of the equation. The conventional wisdom is that it takes $1MM in the bank to generate $40K per annum in perpetuity for retirement without touching the principle. So, just to use easy numbers, a $100K/yr. government pension is worth about $2.5MM. So, again using easy numbers, the private practice radiologist making $500K/yr. would have to save EVERY SINGLE PRE-TAX DOLLAR more that he's making than the $250K/yr. VA radiologist for a decade in order to match the value of the pension. Given how progressive tax brackets work and the tendency of people to adjust their lifestyle to their income, that's extremely difficult to do. Obviously, it's more complicated than that, because the PP radiologist doesn't have to save the full $2.5MM assuming his investments will grow. But on the other side, that pension is pegged to inflation and backed by the full faith and credit of the U.S. government, so that $100K at age 65 could be $130K by age 85.