Are braking vibrations a sign of the end of the life of a wheel? My mountain commuter (2008 Schwinn High Timber) has picked up a vibration in the front wheel under braking.  I'm worried that this is a sign that the rim is end of life.  I checked the V-brakes and they do have some slight play front to back (about a 1 mm at the top of the brake arms), but the bolts for them are still tight.  I have found a few spots on the wheel that looks like a pit, or possibly environmental damage, but not enough of them to justify the vibration.  

Is it possible that a rim will warp into a wavy pattern (I highly doubt it)? 

This rim does not have a wear indicator when new, but will they put something in the rim that will be exposed when worn?

It is a Alex "X101" (best geuss) Rim single wall alloy, but since it is a BSO OEM, it isn't listed on the website.  I have put 15000 km on it with 2/3 of it being daily commuting.   My commute home involves braking from speeds of 50 kph for red lights and a turn on a 3.5 km stretch.  I have to ride the brakes for a short but steep downhill section while escorting the kids home.  The wheel is still true and the bearings are in good shape.  I'm on my 3rd set of brake pads (oem, over the counter bike shop, and now KoolStop dual compounds used for 2000km).  I'm a all weather rider, but I keep the bike clean.  I don't get to ride in nasty slushy snow, but roads are heavily salted here that will linger for weeks.

I'm planning on changing the wheels out anyways here in a few months for other reasons, but I just want to make sure I'm not going to kill myself in the mean time.
Is there any kind of wobble or unevenness in the rim? There shouldn’t be more than about 1mm of wobble. If there is a noticeable wobble the wheel might simply need truing.

Are the brake pads properly aligned? They should wear evenly over the whole surface.

You can measure rim wall thickness with a caliper gauge. A sign for rim wear is also if you can measure a deformation (change in rim width) between zero and maximum tire pressure.