least 2 servers need to be UP in pool, only than VIP is marked as UP

HI F5_LB_Eng , 
You can achieveit without iRules , take a look below : 
F5 Virtual server become UP if its assigned pool is UP "available" , So let’s play and adjust with Pool itself.
I will Start with monitors of pool. 
Let we say you have 5 pool_members with these ips : 
( 1.1.1.1:80 , 1.1.1.2:80 , 1.1.1.3:80 , 1.1.1.4:80 , 1.1.1.5:80 ) 
- Create a specific monitor with Alias option for each pool member ip and port " 1.1.1.1 port 80 , and so on .. " , you should create 5 different monitors for each member , Like below snapshot : 

- After Creating 5 monitors , open ( Local traffic >> pools >> select your pool >> select advanced under Configuration section ) , assign all 5 Specific monitors for this Pool , and change availabilty to " at least 3 " or " at least 2 " based on your needs , that means if you Configure the availability to " at least 2 " , it will marke the whole pool UP if only 2 monitors from 5 monitors successed and the other 3 monitors failed ,  also it will mark the whole pool down if one monitor from 5 monitors successeded and the other 4 monitors failed an so on ... 

To configure the availability , Follow the below Snap shot : 

> I didn’t modified any settings in Virtual server itself but adjusted so configuration in pool monitors so this will reflect with the status of pool at all and this will make you to control in virtual server status easly. 

> Briefly , 
you will control the virtual server status by a group of specific monitors alias and how many monitors will be available at a time.
I played on pool itself as its status controls  virtual server’s Status. 

> Look at the below Snapshots from my Lab , I believe it will help you to solve your concerns : 

So I see the Key for your issue is monitors. 
Try to understand what I mean and try to match it with your environment. 

I hope this solve your inquiry. 

Wouldn't there be a lot of HealthCheck packets?

I don't think this is the right approach. It would allow you to control the pool/VIP state based on the number of available pool members but it could also allow 'failed' members to be marked as UP and being included in LB decisions resulting in connection issues for some clients.

Suppose you have the monitor settings at pool member-level set to 'inherit from pool' which is default settings. All of the monitors on the pool will be ran against each pool member.

1. Let's say you have pool members: 1.1.1.1:80 , 1.1.1.2:80 , 1.1.1.3:80 , 1.1.1.4:80 , 1.1.1.5:80.
2. You create monitors with specific alias address and port for each.
3. You assign all monitors to pool and you set the requirements to 'at least 2' to be successful
4. if port 80 on member 1.1.1.1 becomes unavailable but works fine on other pool members/servers you will still meet the requirements of at least two successful monitors for each pool member, so the pool will keep all members UP incuding the failed one: 1.1.1.1
5. clients who will be forwarded to pool member 1.1.1.1 will however see error/connection issues when the LB algorithm will choose this pool member.