Do you mean they are included in the 1st company roll, or they are just considered veterans?
They wouldn't be included in the 1st Company Roll, because If that was so 1st company thunderhawks and Land Raiders would be detracting from the fighting strength of the company. I'll use my lovely Salamanders as an example.
(Quoted Directly from the Warhammer
40k Wiki,
http://warhammer40k.wikia.com/wiki/Salamanders )
Chapter Fleet
The Salamanders Chapter fleet is known to comprise the following vessels:
◾2 Battle Barges
◾10 Strike Cruisers
◾5 Rapid Strike Cruisers
◾220 Thunderhawk gunships
Now, ignore the Battle Barges and Strike Cruisers. The Salamanders have an exceptional number of Thunderhawks, but if the pilots of the Thunderhawks were to be counted as battle brothers in the standard companies, they would have more than 880 brothers added to the roll. Obviously, you don't mean Thunderhawks, but that's just my example. Not the best, but the first I could think of.
HOWEVER, this would not be the case in other companies. If the entire reserve Assault Company was put formed into Land Speeder squadrons, they would still be considered members of the 8th.
The most basic form of organisation for a standard codex compliant chapter would be 10 Veteran Squads, with of course command squads and all that apparatus. These 10 squads can further be split into 20 Combat Squads, which would seem the logical organisation. If we were to work by basic logic, and also what is common, we would therefore have 10 squads of Sternguard Veterans and 10 squads of Vanguard veterans. We would technically have 5 FULL squads of Vanguard, and 5 FULL squads of Sternguard.
Depending on the situation and foes at hand, the squads could then be turned into
TDA squads or kept as
PA squads.
HOWEVER, if we are to use the
GW 1st Company as an example...
(This is quoted from the Games Workshop website, under Warhammer 40,000/Warhammer 40,000 Armies/Space Marines/Apocalypse Formations/1st Company Veterans.
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat1120011a&prodId=prod2080046a )
1 Captain in Terminator Armour; 1 Chaplain in Terminator Armour; 1 Space Marine Command Squad; 3 Venerable Dreadnoughts; 5 Terminator Squads; 5 Terminator Close Combat Squads; 5 sets of Vanguard Veterans; 3 sets of Sternguard Veterans; and 2 sets of Veterans.
Of course, this is a template, and is not to be taken as exact canon. It provides a good base though.
This entire thread could have been solved by saying 10 Veteran squads, of varying designations and missing space marines (battle losses) which could be split into 20 Combat Squads.