You want to arrive in the UK before your entry clearance starts. Paragraph 30C of the rules sums up what can happen...
30C. An Immigration Officer may cancel an entry clearance which is
capable of having effect as leave to enter if the holder arrives in
the United Kingdom before the day on which the entry clearance becomes
effective or if the holder seeks to enter the United Kingdom for a
purpose other than the purpose specified in the entry clearance.
So the IO has the option, but not the obligation, to have you removed from port (and to fine the carrier). The outcome will depend upon your personal impact and articulation skills plus you will need to set out a tenable rationale of why it's reasonable for you to be in that situation. If the IO buys in to your argument, he can use Paragraph 31A of the rules to make a variation...
31A. Where a person has arrived in the United Kingdom with leave to
enter or remain in the United Kingdom which is in force but was given
to him before his arrival, he may apply, on arrival at the port of
entry in the United Kingdom, for variation of that leave. An
Immigration Officer acting on behalf of the Secretary of State may
vary the leave at the port of entry but is not obliged to consider an
application for variation made at the port of entry. If an Immigration
Officer acting on behalf of the Secretary of State has declined to
consider an application for variation of leave at a port of entry but
the leave has not been cancelled under paragraph 2A(8) of Schedule 2
to the Immigration Act 1971, the person seeking variation should apply
to the Home Office under paragraph 32.
If, on the other hand, the IO concludes that you are abusing the system by attempting to get more leave that you were issued, he may let you in and then make an annotation on your computer file. The annotation would be reviewed the next time you applied for entry clearance.
The last outcome worth considering is where the carrier refuses to board you.
A vignette transfer is where someone's passport expires but there's a visa in it that is still good, so the person may opt to have the visa transferred from his old passport to his new passport. It has nothing to do with your case and they don't transfer vignettes for visitor visas of 6 months or less anyway.