I just had a decent 18th and had a pretty big 16th where im from drinking is legal at 18 so its our version of 21 even though everyone has 21sts anyway. btw 16th: 120 showed with about 180 invited, 18th: 65 showed with 100 invited in both times about 1/10 of the people were friends' guest i met on the night, if ti were anymroe i would hate randoms i dotn know attending my event, limit it to 1 guest p.p. that really needs one i.e. random single girlfriend from work that u cant babysit all night to please them. p.s. rule of thumb is invite 1/3 more people than what you want to attend the event.
Write INVITATION ONLY on your invitations (and even the guest people) for the guests write dear XXXX and guest, you are invited to YYYY's 21st bday etc. you will get people asking to bring guests so allow for it.
I personally woudl never EVER make my guest pay for anything at my celebration (sorry if i offend you) I think its rude to be invited to an event, give a gift and then shell out money. if you want to limit your alcohol allowance there are many ways to go about it without being rude:
1.) have a tab with the bar already set, when it reaches this limit (depending on the price of alcohol at the bar i would suggest 5k) after this point all guests pay their way.
2.) have a time restricted lime limit on drinks i.e. you pay for all drinks up till 12 or 1 am etc.
3.) I strongly suggest this, but maybe its my style because i like my events upper end and classy. have the drinks (and food) served by the staff on trays (cocktail style), no drinks offered via the bar, that way you control the limit and what is available to guests.
4.) Have the available drinks chosen by you in your price bracket anything outside of it is not offered or paid by guests. where im from this is generally wines, champagne, beers and 1 or 2 cocktails.
5.) I don't suggest this but i can be done, have drinks sitting on tables ready when the guests arrive and any additional is at their cost (you can even do a champagne fountain like me) but be aware this can cause problems such as fighting over drinks, loosing drinks, and more importantly drink spiking. you dont want that hanging on your head.
Alcohol is a hard this because it is expensive and some people get as trashed a possible (and can ruin your party) by taking advantage of free grog.
Dont do VIPs and if you do make it subtle. If you really want don't do anything huge like a complete different color scheme as you suggested or roped off areas, they will stick out and other guests will get offended. If your really keen on it i would give my vips something subtle like all matching pendants, broaches or matching twilly scarfs as gifts to wear on the night and if asked they can tell people they were 'gifts' for being good friends or help planning the party etc. (and the staff can differentiate) To tell you the truth i had unintentional vips at my party only because the person is a celebrity and their entourage (who i knew) Which i regret inviting because they got all this unneeded attention and they felt uncomfortable. my parents asked for special better quality nibblees to be given to them by the waiters but this was offered to other guests after the vips had 1st pick. So it wasn't really obvious.
About the cake uh NO! dont make it obvious as that. Here is an alternative to get around it if you want a quality step between 'vips' and guests. Have a small size good quality cake (that you want and ur vips to eat) use this for show and when you blow the candles, have it taken into the kitchen to be cut up and offered to your appropriate guests (don't be strict, if i non-vip guests some don't make a fuss) however have many cheaper cakes reserved in the kitchen to be catered out simultaneously with the initial cake, make sure the icing and color of fillings match and no one will notice and it wont be obvious. it will look like there was more cake cut up in the kitchen not a lower grade. Did that make any sense? or you can have the kitchen make other cheap desserts to pass around i.e. cupcakes and keep the birthday cake to have with your family and friends the next day. you should not discriminate between guests and if you do don't make it obvious.
BTW i just noticed your ticker, you have just under 2 months till ur bday and your at this stage, events like this take months to plan, maybe its not as detailed as mine but FYI it took me 4 months of planing and get a move on with your outfit you need to love ti and not feel pressured to buy something because the big day is coming up you will regret it and really wear the outfit again.
I dont mean to sound rude or offend you in this at all but just the way, i just recently did my party and wanted to advise to you, its your choice and you can do what you like enjoy it its your night!