I've owned a Trek Valencia for many years and have only used the bike a couple of times a year at the most. I recently ventured back into cycling and discovered that the bike is a 20", and I need an 18" (medium). I really like the way the bike rides, but stopping can literally be a pain. This being said I decided to buy a new bike that fit me. I found a great deal on a 2018 Orbea Vector 30. I road the bike and it certainly does fit me better and it seems to also be tighter and smoother.

Question...My Trek has 24 gears, and the Orbea has 16. Where I live is moderately Hilly and my rides round trip at the most are less than 20 miles. All usually in an urbanish area. I'm a total neophyte when it comes to gearing. Will the loss of the 8 gears be a potential negative impact? As an adder I also road a Cannondale Quick 6. The bike was a perfect fit, but the shifting and gearing seemed pretty crappy to me.

You can use a gear range calculator Mike Sherman's Bicycle Gear Calculator to see exactly how the gearing on your current bike compares to the new one. If you plug in the values for wheel size, chainrings, and gears on the cassette you will be able to see things like the upper and lower end for the gearing. If you live in hilly area it's the low end that is most important. I much prefer having enough gears so that I can find a gear for a comfortable cadence regardless of the terrain.

That bike has a 34x34 low gear. That's pretty low and should make climbing hills possible.

The right number of gears is determined by 2 things:
1. How easy do you need your low gear to be and how fast do you want your high gear to be? As I have aged, I have found myself looking for easier low gears and never using my fastest gears. YMMV.
2. How small do you want the steps between gears to be? I like the gears that I use on flat roads to be pretty closely spaced but I like bigger steps between my hill climb gears.

The trike that I ride today has 30 gears. I rarely shift out of my front chainring so I'm pretty sure that I could be happy with just the middle ring.

For a casual rider in a moderately hilly area, you'll be just fine. I ride a similar setup and never find myself wishing for more gears. If you're racing or riding with fast groups, it may become more important to have the extra half steps between gears, but if you're just riding along I don't think you'll have any problems with the new gearing.

no idea about how you are measuring,* horizontal TT 20 and a sloping 18"
can be the same , functionally

as saddle to pedal height is how you ride the bike..

Virtual length of top tube the same?

* Center of BB up seat tube, to center-line of top tube, top of TT

Or top of seat tube extending above top tube, with a collar up there..

Thank you everyone for the information. I see that I have much to learn. I'm pretty excited to be doing that learning on a new bike.

I'm a born and raised Floridian. A 240' hill is a monster!

The height isn't what makes a hill hard, it's the grade. How steep is it?

Unless there is a real need for a suspension fork, my recommendation is to forget about them. They are heavy, as in 4 and 5 pounds heavy and do require maintenance if expected to last the life of the bike. A standard fork is 2-3 pounds at most. No maintenance required on a standard fork. Bonus: They look better than a sus fork!