ALTER procedure [sys].[sp_db_vardecimal_storage_format]
    @dbname sysname = NULL,    -- database name to change
    @vardecimal_storage_format  varchar(3) = NULL    -- vardecimal format to turn on/off
as ...

Great, but this enables the db not the table… a bit more Master DB diving turned up:

ALTER procedure [sys].[sp_tableoption]
    @TableNamePattern      nvarchar(776)
   ,@OptionName            varchar(35)
   ,@OptionValue           varchar(12)
as ...

For reduction estimation use:

ALTER procedure [sys].[sp_estimated_rowsize_reduction_for_vardecimal]
    @table_name nvarchar (776)
as ...

So next thing to find is what option to set:

exec sys.sp_tableoption 'tableName', 'vardecimal storage format', 'on'

so how well does it do? here’s some code:

set nocount on
go
create database varDecimalON
go
create database varDecimalOFF
go
-- enable vardecimal storage format for varDecimalON Dababase
exec sp_db_vardecimal_storage_format 'varDecimalON', 'on'
go
use varDecimalON
if object_id('t1') is not null
    drop table t1

create table t1
(
    col1 decimal(18, 12),
    col2 decimal(18, 12)
)
-- enable vardecimal storage format on t1 in varDecimalON database
exec sys.sp_tableoption 't1', 'vardecimal storage format', 'on'

declare @i int
set @i = 1
-- insert 100000 rows with 2 decimal places
while @i <= 100000
begin
    insert into t1(col1, col2)
    select    convert(decimal(18, 2), rand()*1000000),
            convert(decimal(18, 2), rand()*1000000)
    set @i = @i + 1
end 

set @i = 1
-- insert 100 rows with 12 decimal places
while @i <= 100
begin
    insert into t1(col1, col2)
    select    convert(decimal(18, 12), rand()*1000000),
            convert(decimal(18, 12), rand()*1000000)
    set @i = @i + 1
end
go
select 'space used in varDecimalON Database'
exec sp_spaceUsed 't1', 'true'
/*
-- results for sp_spaceUsed
name    rows      reserved   data        index_size   unused
t1      100100    2632 KB    2592 KB     8 KB         32 KB
*/
go 

use varDecimalOFF
if object_id('t1') is not null
    drop table t1

create table t1
(
    col1 decimal(18, 12),
    col2 decimal(18, 12)
)
declare @i int
set @i = 1
-- insert 100000 rows with 2 decimal places
while @i <= 100000
begin
    insert into t1(col1, col2)
    select    convert(decimal(18, 2), rand()*1000000),
            convert(decimal(18, 2), rand()*1000000)

set @i = @i + 1
end
-- insert 100 rows with 12 decimal places
set @i = 1
while @i <= 100
begin
    insert into t1(col1, col2)
    select    convert(decimal(18, 12), rand()*1000000),
            convert(decimal(18, 12), rand()*1000000)
    set @i = @i + 1
end
go
select 'space used in varDecimalOFF Database'
exec sp_spaceUsed 't1', 'true'
/*
-- results for sp_spaceUsed
name    rows     reserved    data        index_size   unused
t1      100100   2824 KB     2816 KB     8 KB         0 KB
*/

use master
drop database varDecimalON
drop database varDecimalOFF
set nocount off

=========================================================

Zaenkrat se mi še sanja ne, kaj pomeni zgornja koda, vam bom pa znal povedat čez dva meseca :)