Sure looks in the picture like moisture is getting under the vinyl. The most likely cause is the wax seal under the toilet. The two white covers shown in the picture cover the screws that hold the toilet down. You can pry them off with a screwdriver. Try tightening the nuts just a little and see if they are loose. If they are, tighten them, but not too much or you will crack the base of the toilet. If they were loose and you tightened them, wait a week or so and see if the stain gets any worse. If it does, it is probably the wax seal that is bad. As far as getting a plumber....do you have the knowledge to change the wax seal? You can get a "how to" book at HD. The wax seal is only about 59 cents. If you are at all handy and have some tools, you can probably do it.

Be sure to block for grab bars at the toilet and at the tub. The pony wall at the tub, next to the vanity, could be widened to incorporate a pull out for more storage. Lighting, I have one bathroom with 2 sconces at 100 watts each and have wonderful lighting, that room is 6.5x9.5 feet. My other bathroom has 2 sconces with 120 watts each and that lighting is marginal. Check the lighting forum for lumin recommendations. Be sure to have an exhaust fan and enough gap under the doors to allow make up air to enter the room for efficient fan function. I think that is why some people complain about poorly functioning fans even though they are sized correctly, because the room is too tight. Put electric outlet beside the toilet, low, so you can have an advanced toilet seat option. Read my post to Mermanmike that I wrote this morning: Help Laying Out a Master Bathroom...

With toilets, the only consumable is a new wax ring each time you pull it off the flange. I recommend you go beyond the old wax rings and look at the waxless gaskets. They adhere to the bottom of the toilet with peel-and-stick adhesive and once on, you can pull and reset the toilet all you want without having to replace the gasket each time. No issues pulling a toilet. Years ago I had a rare case where I had to pull and replace the only toilet in the house each day for about a week. Pull in the morning, work, replace each night. They can be somewhat heavy. They can be awkward. You don't want to hurt yourself or drop the toilet. But there's nothing remarkable about doing it. If your toilet will be need to be in place during any demo or construction down the road, you could install your new one now and store the old toilet in the basement. Come time for construction, swap them out so if anything bad happens it's the old toiled that gets dinged.

Helen, the driveway bit is what you have to do when you don't do routine maintenance. I've had a toilet on the driveway precisely two times in my lifetime. The first time was when I latched onto the white toilet my neighbor was throwing away because I wanted to replace the avocado green toilet in the powder room. Cleaned it outdoors before installing it. That same toilet was again taken to the driveway when it was removed because I was installing tile in that room, the toilet was out on the porch next to the driveway because that was the closest place to haul it and get it out of the way. Gave it the acid bath because 'why not since it's out here anyway.' Same toilet was removed when I repainted the bathroom this winter but no trip out in sunshine that time because there was about 3' of snow on the ground and the water woulda' frozen in the hose before it got to the john. I wouldn't have to deal with issues of trying to match the footprint of the toilet down the road. ...plus possibly dealing with flooring issues around the silhouette of the old toilet. ??? The flooring is supposed to go under the toilet, not around the toilet. Or are you talking about the flooring possibly being marred? Hasn't been my experience with either vinyl or tile. I wouldn't indulge in false economies and NOT replace a ten year old toilet in a full remodel. False economy, by my definition, would be not upgrading the pipes, drains, shut-off valves, supply hoses, using cheap closet/ flange bolts instead of stainless steel ones, etc. in a remodel. Toilets, OTOH, have an indefinite life and are easily, inexpensively repaired. Being a pay-as-you go DIY'er, reusing the toilet just seems sensible to me. VS trucking to the store, loading a new one on the cart, loading it in the truck, unloading it from the truck and, most likely, sending more American dollars overseas. (Some Totos and some Mansfield porcelain products are made in the USA. Maybe Kohler?). And then being out a tipping fee to dispose of it or having to get out the sledge hammer to break into small enough pieces to sneak into the garbage. But if toilet proprietor X is someone who needs to call for assistance to change a light bulb in the desk lamp, yeh, get a new one, installed. OTOH, if toilet proprietor Y is someone who doesn't know jack squat but managed to put air in a tire, once, he can probably handle repairing a toilet. Bottom line is that the toilet is the first thing out and the last thing in when remodeling a bathroom. So OP has time to think it over. And with a whole lot more information than he bargained for! :-) Have a good day!