Hi Siva,
In customizing (Financial Accounting (New)-> Asset Accounting-> Organizational Structures-> Copy Reference Chart of Depreciation/Depreciation Areas), select posting to GL to your depreciation area 15 as 5 "Area Posts APC Only". This will post your acquisition cost to the depreciation area 15.
About your second question:
This scenario is dealt with under "componentization" of the asset.
Create a main asset with an appropriate primary name like Machinery. Within the master data, Depreciation areas tab, select the depreciation key relevant to 15%. Create another sub-asset within this main asset and name it on Vehicle. This master data can have different characteristics, here in the sub asset master data, enter the depreciation key relevant to 20%.
Regards
Sheli