Installing a vinyl block and attaching an exterior light fixture to it. The one gang wiring box in the exterior wall does not have a cover. Does that one gang wiring box need to be covered before I add the vinyl, lamp-mounting block? Images attached. Please advise.

 

No, but the box is required to be accessible so you will need to cut the siding block the same size as the box. You should also add a box extender as it will be set back from the face of the siding block.

Box extender:https://www.homedepot.com/p/Electric...E1-2/202708650

 

I'm noticing no weatherproofing or even backer board there.
Water gets behind vinyl siding and can run in the wall at that box.

The assemblies used for vinyl siding should come with their own junction box. The idea is that the junction box gets mounted and sealed to the sheathing. Then the box protrudes thru the siding for the cover to snap on.

In the representative picture...... the cable is brought in to the back of the mounting kit using the supplied hole. If the mounting plate with the hole in it fits tightly against the electrical box then that should be ok. Be sure to seal between the sheathing and the back of the plate. Also be sure you have enough wire to come out of the box to connect the light to.

 

Arlington makes my favorite siding blocks/boxes. No box required, just feed the cable into the block.

Arlington | Siding Mounting Blocks

 

T and P: Thank you for your swift responses. I was headed down the wrong path so your suggested course corrections are very helpful.

The vinyl block that I am using matches the siding color and is supplied by NAPCO the siding manufacturer. The vinyl block extends out from the sheathing 1.5 inches [see "200521 mounting side depth 1" below]. The vinyl block has a knock out scored into the vinyl that matches the size of the one gang wiring box. I will remove that knock out and install a wiring box extender that will extend through the vinyl block. I will secure and caulk the wiring box extender to the original wiring box. I will attach the lighting hardware bracket that I currently have attached to the vinyl block to the wiring box extender [see "threaded lighting hardware" below]. I will secure and caulk the vinyl block to the sheathing. I will caulk the wiring box extender where it passes through the vinyl block. I will install the siding [see "vinyl block extends through siding" below] and place the flange. The exterior lamp [see "exterior lamp" below] attaches to the hardware on the wiring box extender. A gasket fits between the lamp base and the vinyl block.

Have I got this correct?

 

Only thing you left out is tear all the siding off and put housewrap on the house. All siding needs a WRB behind it. You can tell it's gotten wet before and it will again.

 

Your plan sounds fine. However, I have found those siding blocks to be a little lacking in strength, and many cases when you tighten the light fixture it might just bend/warp the plastic. You could try it first and see how it works with your flood light.
Instead of a box extender, you could install a 1 1/2" handy box extension or a plastic ring like this: https://www.homedepot.com/p/Smart-Fi...-307424550-_-N If you use a metal handy box extension make sure you bond it somehow to the ground wire. That should give you a more solid mount.