The bulb is on its last legs.

Yes there are CFL based yard lights.

I suppose that you have a sodium vapor lamp that you're intending to replace with a CFL? There are LED lamps too.

Sodium vapor lamps are actually energy efficient, although they are monochromatic.

Most likely if you have an older "vapor" lamp it's mercury vapor, not sodium vapor. Mercury vapor is surprisingly mediocre in efficacy (perhaps 50 l/W at best) and the output declines significantly with time (but not the energy use). The quality of light isn't great (low CRI).

IIRC, because of the low efficacy and lack of tangible benefits, it became illegal to sell mercury vapor ballasts as of 2008. AFAIK you can still buy replacement lamps, if all you want to do is get the rays back. (Sorry to be vague; I don't follow this too closely as I don't have much interest in MV lights.)

Sodium vapor breaks down into two general categories - low pressure sodium and high pressure sodium. I have never seen an LPS fixture offered for home use. Its light really is monochromatic - bright yellow - but its efficacy is spectacular at as much as 200 l/W.

What most people think of as sodium vapor light is high pressure sodium (HPS). This usually has a pinkish-orange light. It's not truly monochromatic, but the CRI (color rendering index) tends to be low (20-30). There are HPS lamps with CRIs in the 65-85 range, but they're more expensive and don't last as long. HPS efficacy is comparable to a good linear fluorescent with an electronic ballast - about 100 l/W.

Fluorescent yard lights I've seen claim as much as ~110 l/W and in my experience have a much better quality of light. I am a little skeptical about these as I suspect they may be playing number games with "phototropic lumen equivalence." So I am going to estimate the efficacy of these at around 80 l/W.

I also haven't seen any fluorescent yard lights recently that I would consider really well made. Maybe someone else has.

Of course you could always fit a PAR-style retrofit CF to a conventional flood fixture.

LED seems to be currently fashionable. It has an (IMO) undeserved reputation of high efficiency based on a few very expensive and largely unavailable high-tech samples. When last I checked, the LED lamps you could actually buy - and which owned up to their actual lumen output - had efficacy no better than (and often worse than) mercury vapor. Their main advantage is in longer life; some manufacturers are now claiming 70% or 80% light output at 50,000 hours.

Maybe David Tay knows something I don't - he seems to be quite a proponent of LEDs - but most of the LED's I've seen for outdoor use have been dim, cheaply made, relatively low efficacy landscape lighting. Many of these are solar powered fixtures. This is actually a good use of LEDs, because they can be made to carry on at nearly full output until the battery is nearly flat. (But so could other types, with comparable circuitry.)

I've run across a few LED flood or spot fixtures that claim equivalence to 100 or 150 watt incandescent. I may have missed some, but I have yet to see one of these which gives a specification for luminous flux output in lumens. I suspect that's because revealing the actual lumen output would show that these floods don't achieve the incandescent equivalence they claim. And they're expensive - prices seem to run around $500 each.

I think that LED area lighting will be a good choice someday, but I have yet to see what I consider a realistic choice for this application. They have a high tech image, however; so they might suit if that's important to you, and you don't mind opening your wallet wide.

For the moment your best bet is probably HPS if you can stand the mediocre color rendering, or fluorescent if you can find a decent quality fixture.

I've also seen fluorescent lamps (bulbs) that are supposed to directly replace mercury vapor lamps (bulbs) in old MV fixtures, using the old MV ballast. These are claimed to deliver higher efficacy and improved CRI, but I don't know how good they really are. If you try one of these, please report back on it.

The following is interesting. I don't have any details on the pricing or actual performance.
http://www.ledlightsorient.com/flood-lights-c-9.html

Should be much better than most LED flood lights from Home Depot / Lowes / Lamps Plus if it works as advertised.

http://www.everlumen.com/ledproducts/led-outdoor-lights/wall-wash-flood-lights/90W-LED-Garage-Light-epid-106.html

Usage: Street Lighting, Parking Lot Lighting, Ramp Lighting, Perimeter Security Lighting, Farm & Ranch Lighting, Loading Dock Lighting, Military Base Lighting.

Probably inappropriate for a residential use.

Thanks for posting that, David. The Everlumen item is impressive.

I note that although it claims an "LED efficiency" of 105 l/W, that apparently doesn't include the power supply. The unit is rated 90W and produces an initial 7600l for about 85 l/W. That's quite good for an LED fixture and nearly comparable to a good fluorescent (100 l/W). At 50,000 hours I'd guess it will still be operating at 59 l/W, better than a mercury vapor at 20,000h but probably not better than a fluorescent at 5,000h.

Regrettably I can't imagine a homeowner installing something as utilitarian-looking as that. No price is given; I hate to think what it might be.

I have to say that "Warranty: Three years products gurantee, five years warranty" is puzzling. That and "Warning: Please use the light in the open fixture" almost screams "made by an English-challenged Chinese company." :-)

As I say, I think LED area lighting has a "bright future" (sorry) but for most applications it's not quite here yet. If I were putting up a yard light right now, it would probably be HPS.

This company makes the LED strip lights that a number of folks have been using. They also have LED flood lights starting at $126 - $180.
30W Flood Light
(Outdoor)
20W Flood Light
(Outdoor)

Part # LBCX1-30
LBCX1-20
ET1-C (110V) ET1-B (240V)
Description
12 x CREE XR-E Cool White 5000~6300K, Warm White 2650~3050K Luminous Flux: 1800 lm for cool white, 1400 lm for warm white CRI: >75 for cool white, >80 for warm white Beam angle: 25/45/90, AC100~240V (DC12/24V optional)

Yet another source for LED flood lights in USA.
http://www.lumecon.com/store