Looking on the internet deeply has found these results:

I was on vacation. is the most popular phrase on the web.

I was on a vacation. 13,540,000 results on the web	I was on vacation. 139,600,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Sure, I could go on a vacation. After you ended everything with us, I went on a vacation. I was just on a short vacation. Because she just wants to go on a vacation. Few days earlier, I was on a vacation with my family. I took few wonderful video clips which were about 20 to 30 minutes each. Boris we're not here on a vacation. Ernie, I'm not going on a vacation. You are not taking my son to Europe on a vacation.	As usual, everything went smoothly during their stay; I welcomed these guests when they arrived, but was unfortunately no there for their departure because I was on vacation. She called me while I was on vacation. Commander Slattery said you were very helpful while I was on vacation. He said, "I was on vacation." I thought I was on vacation. I use to come here when I was home on vacation. I want to start telling people I was on vacation. On the Foussarde Canal, sir. I was there on vacation.

I was on a vacation.

13,540,000 results on the web

I was on vacation.

139,600,000 results on the web

More popular!

Sure, I could go on a vacation.
After you ended everything with us, I went on a vacation.
I was just on a short vacation.
Because she just wants to go on a vacation.
Few days earlier, I was on a vacation with my family. I took few wonderful video clips which were about 20 to 30 minutes each.
Boris we're not here on a vacation.
Ernie, I'm not going on a vacation.
You are not taking my son to Europe on a vacation.

As usual, everything went smoothly during their stay; I welcomed these guests when they arrived, but was unfortunately no there for their departure because I was on vacation.
She called me while I was on vacation.
Commander Slattery said you were very helpful while I was on vacation.
He said, "I was on vacation."
I thought I was on vacation.
I use to come here when I was home on vacation.
I want to start telling people I was on vacation.
On the Foussarde Canal, sir. I was there on vacation.