Does the order of opening in ECO matters, are they placed randomly?

For example, if C24 is Bishop's Opening and C25 is Vienna Game, does that mean they are similar?

They are similar in that they are moves other than Nf3 after 1e4 e5.

Not necessarily.

The Exchange French (C01) and the Advance French (C02) are nothing alike.

It is strictly arranged by move and commonality/number of lines.

For example:

A00-A09 - NOT 1.e4, 1.d4 or 1.c4

A10-A19.- 1.c4 NOT 1...e5 or 1...c5

A20-A29 - 1.c4 e5

A30-A39 - 1.c4 c5

A40-A49 - 1.d4 NOT 1...d5, 1...f5, or 1...Nf6  AND 1.d4 Nf6 NOT 2.c4 

Etc.

Now you have to account for transpositions.  For example:

D10 - D19 is 1.d4 d5 2.c4 c6 (Slav)

D43 - D49 is 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 c6 (Semi-Slav)

So you start the game 1.Nf3 d5 2.c4

You are currently somewhere like A09 I believe.

Black now plays 2...c6.  You will not see this move in A09 at all.  Why?  Because it is a Transposition, and A09 will not tell you that.  You have to know yourself to go to A13 (1.c4 c6 2.Nf3 d5)

White now plays 3.d4.  You will not find this in A13.  You are now in D10-D19 territory, normally reached via 1.d4 d5 2.c4 c6 3.Nf3, which is the same as 1.Nf3 d5 2.c4 c6 3.d4.

Now after 3...Nf6 4.Nc3, EC0 D16-D19 will cover 4...dxc4, but not 4...e6.  You have to go to the D40s for that (Covered under 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 c6).

So in summary.  Neighboring ECO codes are not always similar (See C01 and C02), and you have to know transpositions yourself.  ECO does not guide you.  It is an Encyclopedia, not a study source.