I have the Garmin Edge 520 last week and I read up on about the power meter.

I bought the Cadence and Speed sensor by Garmin.

Do I need the power meter which is the Vector 2? This thing are expensive.

Just to measure my L and R power on the pedals?

If you're at all serious about getting faster and fitter, a power meter is a superb investment.

That said, none of the techs at my LBS have much good to say about the Garmin Vector pedals. Tons of other options out there at or well below the Vector pricepoint.

Will other power pedals work fine on the Garmin Edge 520?

I am still debating if I need power meter and why. :-)

Any current PM with ANT+ wireless will work with your Garmin.
They have a common protocol for discussing power.

I have the PowerPod. Cheap and doesn't work with the crank/pedals. Rainmaker gave it a good review. However it doesn't give you a left/right split lift the Vectors. My son has Vectors and loves them. He races, I ride.

I was initially staunchly anti-PM. Today, it's one of my favorite purchases. I put off getting one for months and months-- until one became available at a price I was (relatively) comfortable with. It takes away all the guessing. I know how far I can go, how long I can maintain a certain level of effort, how many calories I burn on any given day. It lets me know why I'm slow or fast on a ride. Average speed down but power numbers normal? Could be more stoplights, more traffic, more wind that day. Average speed down and power numbers down as well? Maybe fatigue, maybe just me being lazy. Combining the PM in with an HRM allows me to ride waaayyy more than I previously thought possible.

Perceived level of effort can be wildly inaccurate, but the PM won't let you down. Oh, and dual-sided power is nice, but not absolutely necessary... at least not at what it costs for the dual-sided options.

I have Vector 2 and like them very much.

They're down to $1,000 at REI; soon there will be a 20 % off sale.

Any power meter will work with your 520. Only the Vector will be able to tell you things like where on the pedal you apply the torque (center, left, right, etc) but that's really not important on a day-to-day basis. Might help you solve a fit issue, but I can't think of any other use for PCO. I personally don't think PP is useful either. So the Vectors will provide a lot of data to your 520 that other power meters won't, but at the end of the day what really matters is power, and any PM will supply that.

If I had it all to do over, I'd probably get a crank-based PM. I got pedals because you can move them quickly between bikes and they'll go on any bike. There are a lot of different BB standards. Then I bought a gravel bike, and Keo cleats just don't work that well in dirt and mud.

I can see how a power meter can be a worthwhile investment for some people. I can't fault anyone who gets one, and I kinda wish I had one, but I don't think I'll be getting one. I think I have a good enough feel for how hard I went and how strong I am compared with recent times. Maybe I just have good intuition. I believe in calorie counting for controlling one's weight, and I don't do it. I do it intuitively. If I've had enough for the day, I know it. With practice, proper portions just feel right. I'm developing a similar feel for my workout efforts. I rode much harder on Tuesday and yesterday, and I know it. I might back off soon and then get back into my recent level of, uh, vigor. It takes practice.

If I needed to be at a certain fitness level such as racing, I think it would be worthwhile. My current goal is a century ride at the end of October, to complete it without being in too much pain. I'm sure I'll get there without a power meter.