After spending a little more than I expected on my new road bike, I need to save a little on the accessories. So far I got:

Road bike
Pedals
Cycling Shoes
Helmet
Cycling shorts & Jerseys X1
Water cage & bottle X1
5 function basic bike computer
Basic bike multitool

Are their any essentials that I am missing? For example, do I need a vest? or baselayer? or do I have enough for training this summer already?

tube, co2, levers, patch kit

ride!

gloves and glasses

glasses even before helmet

+1 on the eye protection. They don't need to be expensive but they're good from keeping bugs, rocks and birds out of your eyeballs.

frame pump, spare tube, patch kit, floor pump, saddle bag.

+1 on what echotraveler said
+2 on eye protection. You can probably live without gloves unless you intend to ride really long distances or in crappy weather.

If rides of 2 hours or longer are in the plans, add a second water cage and a second bottle.

Depending on your skin type, you may or may not need sunscreen.

And yes, at least one pump is needed. A hand pump would save you money because you can hang it on the bike and you won't have to buy a CO2 inflator and cartridges, and pumping road tires with a hand pump is a good way to build up upper body strength. A floor pump is easier to use, you can get a floor pump with a pressure gauge for about $20 at Walmart.

A basic first aid kit (neosporin, antiseptic wipes, band-aids).

all it takes is one bad fall without gloves to be typing with a straw in your mouth.

2 spare tubes. Flats happen.
Computer would be at the bottom of my list.

-socks
-another cage/bottle
-more bibs/jerseys
-waterproof jacket
-lights
-lube
-solvents
-epo

1+

Exchange flat fixing stuff (spare tube and pump) for the bike computer and then only buy other stuff on a need-to basis. Also, glasses are good, and lots of guys on a budget use safety glasses you can buy for $10 at Home Depot for eye protection. There's lots of stuff you will probably end up with if you stay in the sport long enough, but beyond what you've listed and suggestions for flat repair and eye protection, you are set for the time being.

hshshshsaahahahahhahahahahahah

Contents of my seat bag:
- Spare tire
- Patch kit
- CO2 inflator
- CO2 cartridge
- Presta adapter
- Multitool (Park Tool MTB-7)
- Small roll of duct tape
- Tube of antibiotic ointment
- Band-aids
- Antiseptic wipes
- Tylenol (4x 500 mg pills)
- $20 bill

once you start buying stuff it never stops.... and what may be non-essential now becomes essential in the cold weather.
socks
leg warmers
arm warmers
cold weather hat
shoe covers
tights
full finger gloves

cell phone is important.

I used to do this. Now all I have is a spare tube, a set of tire irons, a spoke wrench and a multitool.

My priorities:

1. bottle cage + bottle. If you're in a hot place, times 2 or 3 or get a camelback
2. bus fare (and stay on bus routes) OR patch kit + levers + pump (spare tube{s** + spare tire for really long rides.
3. multi-tool
4. glasses
5. helmet

Everything after that is just niceties. I still ride my Pinarello with clips/straps/running shoes. Regular shorts and shirts work fine too.

Maybe switch #1 and #2. You can ride from water fountain to water fountain if you plan your rides right.

At the very least you also need CO2 or a frame pump. (What good is a spare tube if you have no way to inflate it?) I go with CO2 because I don't have any place to hang the pump, I have bottle holders everywhere. My multitool includes tire irons and spoke wrenches. The first-aid kit comes useful sometimes, as does duct tape.

The only things left are the presta adapter (tiny and light), the patch kit (light), and the $20 bill.

You forgot a pair of disposable gloves (latex or nitrile) so you don't get grease all over your bike after fixing a flat.

How would I get grease on my hands from fixing a flat? Worst case scenario, I'd have to touch the chain and the rear derailer once or twice to take off the rear wheel (if that's the one I'm fixing).

Air horn & tank, windscreen, big fat side mirrors, leather panniers, roll bar, etc...

this is all a little much if you're just starting out and on a budget. i still ride my CAAD with cages/toe clips/running shoes. i only have one jersey which i never wear.

the only stuff i consider "essentials" are a helmet, pump (since only bike shops will have presta), and my little ziploc bag which contains a spare tube, multi-tool (which has tire levers). oh, and i ride at night a decent amount so lights, front and rear. i keep this stuff in the spare pockets of my camelbak.

i HAVE some of the other stuff... a spoke wrench, glasses, gloves, bike computer, etc. but i'd hardly call them essentials.

i'd say as long as you're safe (helmet, hydration) and ready in case of a problem (pump/spare tube, or at least a cell phone for a pickup), worry about the rest later.

Ditto Nick's post above. When I was a kid in the 60s and 70s my friends and I had some great adventures on our old bikes with no "gear" at all, other than the bike. I walked home with a flat once and spent a few bucks on a patch kit, tire levers, and a pump. I gradually built up my kit over the years with specialized tools and clothing, as my trips got longer, my skills improved, and/or I saw the need. I've never spent much money on the sport and have always had a great time.