First, stay away from the fancy microprocessor powered complex type. Essentially anytime you REALLY need to charge a battery they refuse to do it. Get a good manual, non-electronic type. For example I bought a Vector brand $100+ charger a few years back. The only battery it would charge is one that didn't really need it. I gave it away to a family member victim. BY FAR the best battery charger I have ever seen or known of is a Schumacher SE 125A. Bought mine 9 years ago and it cannot be beat. Amazon says they are no longer available. It may be hard to find a decent battery charger these days!
Best I find in 10 min search is the Westward brand "benchtop" 1JYU6A charger which will handle both 6 and 12V batteries, gives a 15A so called fast charge and 100A momentary "start" setting or "Boost current" and they are $106 on Amazon. Those specs are not quite as high/good as my old Schumacher but would do most charging chores well. No idea what charging voltage they use but 13.8v is pretty standard.
I have zero experience with AGM batteries but they are lead-acid with glass mat separators, etc. Bragging about deep cycle ability, etc. I see no reason why any good battery charger would not work with them.
At one point, Shumacher was a leading brand in the battery charger business. Sadly, that does not still seem to be the case. They may still make some high end models (if so, I have not used them), but they also make some junk.
I'm going to have to disagree with that blanket statement about smart chargers. A QUALITY smart charger will do much more to extend the life of your battery and protect it from overcharging (something that is often a problem with the dumb chargers). Unfortuntely, just because a charger is mcroprocessor controlled, does not mean it is a quality charger. Some of of them are still junk: some don't last, some use improper charging and float voltages even when brand new. Some lack temperature compensation or a desulfating feature.
I do know that many smart chargers will not attempt to charge a battery if it is below a certain threshhold voltage. (One reason for this is that it makes it possible to protect a charger if the leads are shorted out: if it does not detect a voltage, it will not start charging.) Some of them will not attempt a charge if the battery is below 10 volts (that was common years ago when smart chargers were relatively new. Some of them still use that threshhold today). Many modern smart chargers us a much lower threshhold, such as 3 volts, or incorporate a "force mode" where the charger will act like an old "dumb charger" and start charging even if it detects no voltage. If you are trying to charge a dead battery with a smart charger that lacks these features, one trick is to charge them with a dumb charger first, until they are up to a voltage that the smart charger can detect.
I'm not familiar with the "Vector" brand of chargers, nor can I find any independent reviews of them. Even Amazon seems to have no reviews on the one model Vector charger they do sell. (That seems odd for a company that claims: "You can put your confidence in because we have been delivering quality products for over 25 years.") It's possible you got a bad unit, or that the batteries you were trying to charge were below its threshhold for charging.
I just recounted: I own seven BatteryMINDer chargers. My oldest is about 20 years old and still working well. The newer ones work better and have more advanced features, but I'm still using that original one to maintain the battery in an antique ATV that is seldom used. It cranks the engine over no matter how long the battery has been sitting. Others maintain my 24 aircraft battery, the AGM battery in my tractor, a Gel battery in a motorcycle, the deep cycle sealed flooded lead acid battery that runs the hydraulics on my dump trailer, and AGM in my daughter's ATV. and an AGM motorcycle sized battery that runs the remote control receiver and solenoids for the hydraulics on my log forwarding trailer (there is no charging system on the manual start engine on that trailer, so this battery is cycled deeply at times.) Some of these batteries are 10 years old and still working just fine. The company also stands behnd their products: The display board blew out on my aviation charger when a nearby lightning strike caused a number of issues with electrical equipment in the 10-bay aircraft hangar. The unit was over 4 years old. I had lost my receipt, but they still replaced the board under their 5 year warranty based on the manufacturing date of the unit being right around 5 years prior.
The Noco Genius two-bank charger I installed in my antique truck with a 24 electrical system has also worked flawlessly. I was warned when I bought the truck that the batteries were well past their prime, and I did notice that they self-discharged more rapidly than any of my other batteries. Since leaving them on the Noco charger regularly, the batteries have been restored to almost new condition. I've gotten another 2.5+ years out of batteries that the previous owner was ready to throw out.
If you are convinced that all smart chargers are junk, I doubt I'll convince you otherwise. I'm just relating my experience with these two brands, and the extensive research I did on charging profiles, quality, and reliability of various chargers before settling on these two brands. None of my smart chargers are high amperage (the largest maxes out at 10 amps). I use them more as maintainers, other than the ones used on my dump trailer and log trailer, which tend to get deep cycled. I could get by with far fewer chargers, but I I've accumulated enough of them over the years, that I just leave them dedicated to the equipment on which they are installed. I do own a dumb charger with a 50 amp boost feature for starting, but I don't use it much.
